Fix regex for capturing category names

The previous regex would occasionally miss categories. For example, compare [en.coffee](https://github.com/codecombat/codecombat/blob/ae7a2dfe6af42f0c99f9286841ef641f8c0ab081/app/locale/en.coffee#L100) and [de-DE.coffee](https://github.com/codecombat/codecombat/blob/ae7a2dfe6af42f0c99f9286841ef641f8c0ab081/app/locale/de-DE.coffee#L100).