Enable pypy tests in .travis.yml
Purge use_errno argument from function prototype calls

- It is already contained in prototype definition.
- PyPy is not able to run code with use_errno there.
Only install mercurial and bazaar if using CPython
Set use_mercurial and use_bzr to False in PyPy
Merge pull request #909 from ZyX-I/pypy-support

Enable pypy tests in .travis.yml