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.
Refactor test suite to use expect syntax

Use the expect syntax in favor of the should syntax consistently
throughout HighVoltage.

Removes any 'should' prefixes in `it` descriptions.

Require expect syntax
Deprecated caching

Deprecate caching because page and action caching were removed from
Rails 4. Also, caching can be achived easily by overrideing the
HighVoltage pages controller.
Update NEWS to include latest changes
Require active_support string core ext
Fix #136 Cannot define multiple included blocks for a Concern
Ruby 2.1.0 to travis test suite
Update travis to test against Ruby 2.1