Add instruction for overriding layouts for all static pages

There's nothing in the documentation yet about changing the layout for all static pages so I added it.
Merge pull request #135 from daryllxd/patch-1

Add instruction for overriding layouts for all static pages
Handle bundle install failures with travis_retry

Resolve timeout issues when running bundle install on Travis by using
the `travis_retry` command. It will retry the install command up to 3
times.