Cannot install ruby hooks on OS X
I can install pre-commit fine, but when it comes to running it for the first time, I get this error:

```
[INFO] Installing environment for git@***/pre-commit/mirrors-scss-lint.
[INFO] Once installed this environment will be reused.
[INFO] This may take a few minutes...
An unexpected error has occurred: CalledProcessError: Command: ['bash', '-c', '. /Users/markl/.pre-commit/repoLPZw60/rbenv-1.9.3-p484/bin/activate && cd /Users/markl/.pre-commit/repoLPZw60/ && gem build *.gemspec && gem install --no-document *.gem']
Return code: 1
Expected return code: 0
Output:
      Successfully built RubyGem
      Name: __fake_gem
      Version: 0.0.0
      File: __fake_gem-0.0.0.gem

Errors:
    WARNING:  no email specified
    WARNING:  no homepage specified
    WARNING:  description and summary are identical
    ERROR:  While executing gem ... (OptionParser::InvalidOption)
        invalid option: --no-document


Check the log at ~/.pre-commit/pre-commit.log
```

Environment details:
OS X 10.10.5
`gem --version`: 2.4.8
`ruby --version`: ruby 2.0.0p481 (2014-05-08 revision 45883) [universal.x86_64-darwin14]

diff --git a/pre_commit/languages/ruby.py b/pre_commit/languages/ruby.py
index 7b018c1..0931a58 100644
--- a/pre_commit/languages/ruby.py
+++ b/pre_commit/languages/ruby.py
@@ -89,7 +89,7 @@ def install_environment(repo_cmd_runner, version='default'):
                 _install_ruby(ruby_env, version)
             ruby_env.run(
                 'cd {prefix} && gem build *.gemspec'
-                ' && gem install --no-document *.gem',
+                ' && gem install --no-ri --no-rdoc *.gem',
             )
 
 
['pre_commit/languages/ruby.py']