Set `viewport` to `initial-scale=1`

This sets the content's initial scale to 1 for mobile devices. Without
this tag, devices will load the page zoomed out to fit the entire
content on the screen. We need this when using a responsive grid
framework such as Bourbon Neat.

https://developer.apple.com/library/ios/documentation/appleapplications/reference/safariwebcontent/usingtheviewport/usingtheviewport.html
https://developer.mozilla.org/en-US/docs/Mozilla/Mobile/Viewport_meta_tag