Switch to a different version of Xcode and set travis-tests to fail on error.

.travis-tests.sh

@@ -1,5 +1,7 @@
 #!/bin/bash
 
+set -e
+
 export CC=$CC_OQS
 
 autoreconf -i
@@ -34,5 +36,3 @@ make clean
 make
 make test
 for f in $(ls .travis/*-check.sh); do bash $f; done
-
-

.travis.yml

@@ -46,11 +46,13 @@ matrix:
     before_install:
     - sh .travis/install-clang-format-linux.sh
   - os: osx
+    osx_image: xcode8.3 # https://docs.travis-ci.com/user/reference/osx/
     compiler: clang
     env: CC_OQS=clang
     before_install:
     - brew install https://raw.githubusercontent.com/Homebrew/homebrew-core/0c4314c499576b28e4c082b591228a8f940954c0/Formula/clang-format.rb
   - os: osx
+    osx_image: xcode8.3 # https://docs.travis-ci.com/user/reference/osx/
     compiler: clang
     env:
     - CC_OQS=clang