Revert "ignore the _build directory"

This reverts commit 1a9fe2d8a423a1d108c7fba10c49c677ec6b515d.
Fix path to style.css
fix about page link
_build directory is needed to run on Heroku
Merge branch 'master' of github.com:elixir-dallas/elixir-dallas-web
mix clean && mix compile
mix deps.clean && mix deps.get && mix deps.compile