Fix viewport meta tag

Google, [Mozilla](https://developer.mozilla.org/en/docs/Mozilla/Mobile/Viewport_meta_tag) and [Apple](https://developer.apple.com/library/mac/documentation/AppleApplications/Reference/SafariWebContent/UsingtheViewport/UsingtheViewport.html) use a colon: `content="width=device-width, initial-scale=1"`

It also make [Google PageSpeed Insights](https://developers.google.com/speed/pagespeed/insights/) happy:
- Before
![Google PageSpeed Insights before](http://s21.postimg.org/afa06pcxj/fix_viewport_before.png)

- After
![Google PageSpeed Insights after](http://s21.postimg.org/stkjaop87/fix_viewport_after.png)