Fix `using` keyword highlighting

Only the first instance per file was being highlighted properly e.g.

![](http://cl.ly/image/1C2t252R1Q2C/Image%202014-02-27%20at%2011.50.55%20am.png)

Fixed version

![](http://cl.ly/image/3c0p0X1d3u33/Image%202014-02-27%20at%2012.36.49%20pm.png)

Based off how the language-java package highlights `import`