Avoid installing HTTP libraries with native extensions for non-MRI

In JRuby they don't work, and in Rubinius they have some problems as
well so we already skip testing a bunch of them in code.

Gemfile

@@ -5,3 +5,10 @@ gem "faraday", ">= 0.9.2"
 gemspec
 
 gem 'jruby-openssl', :platforms => :jruby
+
+platform :mri do
+  gem "typhoeus"
+  gem "patron"
+  gem "em-http-request"
+  gem "curb"
+end

Gemfile.faraday-0.8

@@ -3,3 +3,8 @@ source "https://rubygems.org"
 gem "faraday", "~> 0.8.8"
 
 gemspec
+
+platform :mri do
+  gem "typhoeus"
+  gem "patron"
+end

spec/monkey_patches.rb

@@ -1,5 +1,3 @@
-require 'typhoeus'
-
 module MonkeyPatches
   extend self
 
@@ -134,10 +132,16 @@ def realias_net_http(alias_extension)
 # for WebMock to work with them.
 require 'httpclient'
 
-unless RUBY_INTERPRETER == :jruby
-  require 'patron'
-  require 'em-http-request'
-  require 'curb'
+if RUBY_INTERPRETER == :mri
+  require 'typhoeus'
+  begin
+    require 'patron'
+    require 'em-http-request'
+    require 'curb'
+  rescue LoadError
+    # these are not always available, depending on the Gemfile used
+    warn $!.message
+  end
 end
 
 if defined?(::Typhoeus.before)

vcr.gemspec

@@ -39,10 +39,6 @@ Gem::Specification.new do |spec|
   spec.add_development_dependency "timecop"
   spec.add_development_dependency "multi_json"
   spec.add_development_dependency "json"
-  spec.add_development_dependency "typhoeus"
-  spec.add_development_dependency "patron"
-  spec.add_development_dependency "em-http-request"
-  spec.add_development_dependency "curb"
   spec.add_development_dependency "yajl-ruby"
   spec.add_development_dependency "relish"
   spec.add_development_dependency "mime-types"