Fixes for Python 3 CI

[Amar1729]

This PR is meant to serve as a reference while py3 support is being worked on in master.

Updated a few things from https://github.com/pyroscope/pyrocore/pull/63 to see whether I could get it building nicely in Travis:

• added coverage to requirements-dev
  • reason: other PR's paver test step was failing because coverage wasn't installed, I have no clue why it's working for py2 though
• removed six from requirements
  • reason: six doesn't typically need to be a top-level requirement of python projects, esp. in this case it's transitively depended on by other requirements anyway
• updated pyrobase requirement to git url
  • reason: master is newer than the current release and has py3 updates
• update .travis.yml (minor changes)

[Amar1729]

Huh. so here's the current (successful!) build for this PR in travis, I have no clue why but the build status isn't showing up next to my commits.