First clone the ruby TextMate bundle somewhere. The path ~/.idea is just one I picked, you can clone it anywhere. You'll point PHPStorm to this folder later.
git clone https://github.com/textmate/ruby.tmbundle
Then, open up PHPStorm preferences to enable that plugin. Preferences > Editor > TextMate Bundles. Click the + icon to add the cloned bundle.
Note: TextMate bundles have their own syntax highlighting rules separate from PHPStorm. You'll need to edit the colors by selecting from the list below the + icon. The mapping is a 1-to-1 PHPStorm theme to TextMate theme.