Fixed Caching and Cleaned up Old Caching

Caching was broken for many reasons:
* Because puma calls `Travis::Web::App.initilize` multiple times. This caused
the server start time to be incorrect, breaking caching.
* The `Date` HTTP header was missing. This caused some browsers to fail to cache
assets.
* The `ETag` was also incorrectly formatted.

Some other things which were changed:
* Removed `Rack::Cache`
Converted image assets to svg

Thanks to font awesome for the amazing icons:

Font Awesome by Dave Gandy - http://fontawesome.io