Patch ActiveRecord::Schema#enable_extension

In sqlite3 and mysql2 connection adapter, this method is not
supported. Ignore in adapters that does not support it.
Enable Travis CI sqlite3 and mysql2 test
Add gem 'poltergeist'.

It is a capybara javascript driver using phantomjs
Use poltergeist as capybara js driver if phantomjs is found.

Firefox driver steal mouse, which is disturbing in development. And use
poltergeist in CI can remove the dependencies on X.
Remove X dependencies from .travis.yml
Merge pull request #479 from doitian/ignore-enable-extention-in-adapter-not-support-it

Fix mysql and sqlite schema load
Merge branch 'use-poltergeist-in-test-by-default-when-phantomjs-is-found' of github.com:doitian/19wu into doitian-use-poltergeist-in-test-by-default-when-phantomjs-is-found
merge master.