Rewrite gemfile_ruby guide from HAML to Markdown (#1058)

Signed-off-by: Takuya Noguchi <[email protected]>

source/guides/gemfile_ruby.html.md

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+## Specifying a Ruby Version
+
+Like gems, developers can setup a dependency on Ruby.
+This makes your app fail faster in case you depend on specific features in a Ruby VM.
+This way, the Ruby VM on your deployment server will match your local one. You can do this by using the `ruby` directive in the `Gemfile`:
+
+~~~ruby
+ruby 'RUBY_VERSION', :engine => 'ENGINE', :engine_version => 'ENGINE_VERSION',
+:patchlevel => 'RUBY_PATCHLEVEL'
+~~~
+
+If you wanted to use JRuby 1.6.7 using Ruby 1.9.3, you would simply do the following:
+
+~~~ruby
+ruby '1.9.3', :engine => 'jruby', :engine_version => '1.6.7'
+~~~
+
+It's also possible to restrict the patchlevel of the Ruby used by doing the following:
+
+~~~ruby
+ruby '1.9.3', :patchlevel => '448'
+~~~
+
+Bundler will make checks against the current running Ruby VM to make sure it matches what is specified in the `Gemfile`. If things don't match, Bundler will raise an Exception explaining what doesn't match.
+
+~~~
+Your Ruby version is 1.8.7, but your Gemfile specified 1.9.3
+~~~
+
+Both `:engine` and `:engine_version` are optional.
+When these options are omitted, this means the app is compatible with a particular Ruby ABI but the engine is irrelevant.
+When `:engine` is used, `:engine_version` must also be specified.
+Using the `platform` command with the `--ruby` flag, you can see what `ruby` directive is specified in the `Gemfile`.
+
+~~~
+ruby 1.9.3 (jruby 1.6.7)
+~~~
+
+<a href="/man/bundle-platform.1.html" class="btn btn-primary">Learn More: bundle platform</a>
+
+In the `ruby` directive, `:patchlevel` is optional, as patchlevel releases are usually compatible and include important security fixes.
+The patchlevel option checks the `RUBY_PATCHLEVEL` constant, and if not specified then bundler will simply ignore it.
+Version operators for specifying a Ruby version are also available.
+The set of supported version operators is that of Rubygems (`gem` version operators). (ie. `<`, `>`, `<=`, `>=`, `~>`, `=`)
+
+~~~ruby
+ruby '~> 2.3.0'
+~~~
+
+<a href="https://guides.rubygems.org/patterns/#declaring-dependencies" class="btn btn-primary">Learn More: Version Operators</a>