Remove multi_json dependency by requiring Ruby 1.9

This was the last lingering dependency. I was able to convert almost
every method to request and parse YAML instead of JSON but I got stuck
on rubygems/gemcutter#323. Ruby 1.9 includes JSON as part of the
standard library, so by requiring 1.9, I don't need to specify it as a
dependency. Users of older Ruby versions can hack around this constraint
by force-installing the gem and requiring a JSON library manually.

Closes #4.

.travis.yml

@@ -1,7 +1,6 @@
 rvm:
-  - 1.8.7
+  - 1.9.1
   - 1.9.2
-  - jruby
-  - rbx
-  - ree
+  - 1.9.2-head
+  - 1.9.3-head
   - ruby-head

gems.gemspec

@@ -9,18 +9,18 @@ Gem::Specification.new do |gem|
   gem.add_development_dependency 'simplecov', '~> 0.4'
   gem.add_development_dependency 'webmock', '~> 1.6'
   gem.add_development_dependency 'yard', '~> 0.7'
-  gem.add_runtime_dependency 'multi_json', '~> 1.0.3'
 
-  gem.authors       = ["Erik Michaels-Ober"]
-  gem.email         = ['sferik@gmail.com']
-  gem.description   = %q{Ruby wrapper for the RubyGems.org API}
-  gem.summary       = gem.description
-  gem.homepage      = 'https://github.com/sferik/gems'
+  gem.authors               = ["Erik Michaels-Ober"]
+  gem.email                 = ['sferik@gmail.com']
+  gem.description           = %q{Ruby wrapper for the RubyGems.org API}
+  gem.summary               = gem.description
+  gem.homepage              = 'https://github.com/sferik/gems'
 
-  gem.executables   = `git ls-files -- bin/*`.split("\n").map{ |f| File.basename(f) }
-  gem.files         = `git ls-files`.split("\n")
-  gem.test_files    = `git ls-files -- {test,spec,features}/*`.split("\n")
-  gem.name          = 'gems'
-  gem.require_paths = ['lib']
-  gem.version       = Gems::VERSION
+  gem.executables           = `git ls-files -- bin/*`.split("\n").map{ |f| File.basename(f) }
+  gem.files                 = `git ls-files`.split("\n")
+  gem.test_files            = `git ls-files -- {test,spec,features}/*`.split("\n")
+  gem.name                  = 'gems'
+  gem.require_paths         = ['lib']
+  gem.required_ruby_version = '>= 1.9'
+  gem.version               = Gems::VERSION
 end

lib/gems/client.rb

@@ -1,7 +1,7 @@
 require 'date'
 require 'gems/configuration'
 require 'gems/request'
-require 'multi_json'
+require 'json'
 require 'yaml'
 
 module Gems
@@ -49,7 +49,7 @@ def search(query)
     #   Gems.versions 'coulda'
     def versions(gem_name)
       response = get("/api/v1/versions/#{gem_name}.json")
-      MultiJson.decode(response)
+      JSON.parse(response)
     end
 
     # Returns the total number of downloads for a particular gem