Use execute resource rather than the python_pip LWRP to install Sentry.

Because of https://github.com/poise/python/issues/98 we cannot install the
development version from python_pip.
Merge remote-tracking branch 'origin/master' into do-execute-over-pip

Conflicts:
	recipes/_install.rb
Merge pull request #3 from pilbot/do-execute-over-pip

Use execute resource rather than the python_pip LWRP to install Sentry.