(maint) Add Travis CI Support

This patch adds Travis support.  It also makes sure that `bundle install
--path vendor` works with `bundle exec rake spec`.

This patch should coincide with enabling Travis CI support for pull requests.
A build status image is also included in the project README.
Future proof authstore ruby issue 7477

Without this patch once a new patch level version of MRI comes out,
Travis CI will start failing because the test that fails on
`IPAddr.new('::2:3:4:5:6:7:8')` because of ruby issue 7477 [1].  This
patch fixes the problem by raising an exception if the bugged code is
still bugged.  When the issue is fixed the blocks of code that cause the
examples to be marked as pending will pass without exception.

[1]: http://goo.gl/Bb1LU