Fix ruby quoting style to match used by hound.

Fix it to the current enforced style as seen at: https://github.com/gitlabhq/gitlabhq/pull/7233#discussion_r14397848

Did Hound change the default quoting style or did this project change some configuration? It was enforcing single quotes only 5 days ago: https://github.com/gitlabhq/gitlabhq/pull/7211/files#diff-fd1bf4337c05855a90dd234bf800e9e6R25