Ensure exit code is propagated.

If `stop` raises and rescues an exception, $! becomes nil. Seemingly impossible
to reproduce in a unit test. For more information, see
https://github.com/teampoltergeist/poltergeist/issues/497

Fixes #497.
Merge pull request #498 from botandrose/fix_exit_code

Ensure exit code is propagated.
We'll have to implement features provided by capybara >= 2.3
Close dup'ed fd after using
Show off the latest release
Initial pack of changes for capybara 2.3 compatibility
Speed up travis runtime.

Vast majority of time is in bundle install, and the vast majority of
that is installing nokogiri. Parallelize bundle install, and use system
libraries for nokogiri installation.