Require zlib for Ruby 1.8

In Ruby 1.9, zlib is required by net/http but this is not the case in
Ruby 1.8, causing a NameError (uninitialized constant
Faraday::Adapter::NetHttp::Zlib).

lib/faraday/adapter/net_http.rb

@@ -4,6 +4,7 @@ rescue LoadError
   warn "Warning: no such file to load -- net/https. Make sure openssl is installed if you want ssl support"
   require 'net/http'
 end
+require 'zlib'
 
 module Faraday
   class Adapter