Run sentry server with python2

- Rationale: python-cherrypy3 is available in the archives for both
  precise and trusty, whereas python3-cherrypy3 is not, and we want
  to be able to install everything from the archives.

- Updated Makefile to run sentry tests with both py3 and py2

Signed-off-by: Tim Van Steenburgh <tvansteenburgh@gmail.com>