Cache Bundler dependencies on Travis CI

- Even after the cache is restored, `bundle install` still needs to hit
  RubyGems.org because there is no Gemfile.lock. There doesn't seem to
  be a way to tell it that everything it needs is right there in the
  `./bundle` directory.

- Can't use `--standalone` due to bundler/bundler#2851

- It's not necessary to manually update RubyGems and Bundler anymore.

.gitignore

@@ -13,6 +13,7 @@ bin
 *.gem
 .bundle
 Gemfile.lock
+/bundle
 
 ## PROJECT::SPECIFIC
 .rbx

.travis.yml

@@ -1,14 +1,5 @@
-before_install:
-  - gem update bundler
-  - bundle --version
-  - gem update --system 2.1.11
-  - gem --version
-bundler_args: --standalone --without development
-script: RUBYLIB=./bundle script/test
-
-env:
-  - SSL=no
-  - SSL=yes
+install: script/cached-bundle install --path ./bundle --without development -j 3 -r 3
+script: bundle exec script/test
 
 language: ruby
 
@@ -33,3 +24,12 @@ rvm:
   - jruby-19mode
   - jruby-head
   - rbx
+
+env:
+  matrix:
+    - SSL=no
+    - SSL=yes
+  global:
+    - AMAZON_S3_BUCKET=ci-cache
+    - AMAZON_ACCESS_KEY_ID=AKIAJQCVTDEWQHRPBPGQ
+    - secure: "fgk+vBuCp9GW2AlNAcWy5Axl+UEjXTTCwZa6boa5lvg6yBaBKHxqT4XH31U5My3SxJEaQpPDNE/ObHv8InV9DOP5ZI7c8s3cVrBx5a8LZSd25UZBozUHxMeZmFGlgc3syYkLjoeAE+4YmhmTnuKncb0xhke90wJw5T4Z5+GPmzI="

script/cached-bundle

@@ -0,0 +1,46 @@
+#!/usr/bin/env bash
+# Usage: cached-bundle install --deployment
+#
+# After running `bundle`, caches the `./bundle` directory to S3.
+# On the next run, restores the cached directory before running `bundle`.
+# When `Gemfile` changes, the cache gets rebuilt.
+#
+# Requirements:
+# - Gemfile
+# - TRAVIS_REPO_SLUG
+# - TRAVIS_RUBY_VERSION
+# - AMAZON_S3_BUCKET
+# - script/s3-put
+# - bundle
+# - curl
+#
+# Author: Mislav Marohnić
+
+set -e
+
+compute_md5() {
+  local output="$(openssl md5)"
+  echo "${output##* }"
+}
+
+download() {
+  curl --tcp-nodelay -qsfL "$1" -o "$2"
+}
+
+bundle_path="bundle"
+gemfile_hash="$(compute_md5 <"${BUNDLE_GEMFILE:-Gemfile}")"
+cache_name="${TRAVIS_RUBY_VERSION}-${gemfile_hash}.tgz"
+fetch_url="http://${AMAZON_S3_BUCKET}.s3.amazonaws.com/${TRAVIS_REPO_SLUG}/${cache_name}"
+
+if download "$fetch_url" "$cache_name"; then
+  echo "Reusing cached bundle ${cache_name}"
+  tar xzf "$cache_name"
+fi
+
+bundle "$@"
+
+if [ ! -f "$cache_name" ]; then
+  echo "Caching \`${bundle_path}' to S3"
+  tar czf "$cache_name" "$bundle_path"
+  script/s3-put "$cache_name" "${AMAZON_S3_BUCKET}:${TRAVIS_REPO_SLUG}/${cache_name}"
+fi

script/s3-put

@@ -0,0 +1,71 @@
+#!/usr/bin/env bash
+# Usage: s3-put <FILE> <S3_BUCKET>[:<PATH>] [<CONTENT_TYPE>]
+#
+# Uploads a file to the Amazon S3 service.
+# Outputs the URL for the newly uploaded file.
+#
+# Requirements:
+# - AMAZON_ACCESS_KEY_ID
+# - AMAZON_SECRET_ACCESS_KEY
+# - openssl
+# - curl
+#
+# Author: Mislav Marohnić
+
+set -e
+
+authorization() {
+  local signature="$(string_to_sign | hmac_sha1 | base64)"
+  echo "AWS ${AMAZON_ACCESS_KEY_ID?}:${signature}"
+}
+
+hmac_sha1() {
+  openssl dgst -binary -sha1 -hmac "${AMAZON_SECRET_ACCESS_KEY?}"
+}
+
+base64() {
+  openssl enc -base64
+}
+
+bin_md5() {
+  openssl dgst -binary -md5
+}
+
+string_to_sign() {
+  echo "$http_method"
+  echo "$content_md5"
+  echo "$content_type"
+  echo "$date"
+  echo "x-amz-acl:$acl"
+  printf "/$bucket/$remote_path"
+}
+
+date_string() {
+  LC_TIME=C date "+%a, %d %h %Y %T %z"
+}
+
+file="$1"
+bucket="${2%%:*}"
+remote_path="${2#*:}"
+content_type="$3"
+
+if [ -z "$remote_path" ] || [ "$remote_path" = "$bucket" ]; then
+  remote_path="${file##*/}"
+fi
+
+http_method=PUT
+acl="public-read"
+content_md5="$(bin_md5 < "$file" | base64)"
+date="$(date_string)"
+
+url="https://$bucket.s3.amazonaws.com/$remote_path"
+
+curl -qsSf -T "$file" \
+  -H "Authorization: $(authorization)" \
+  -H "x-amz-acl: $acl" \
+  -H "Date: $date" \
+  -H "Content-MD5: $content_md5" \
+  -H "Content-Type: $content_type" \
+  "$url"
+
+echo "$url"