add pypy tests for travis
allow pypy travis test failures
tests print implementaion details in header
handle missing platform.python_implementation in CPython 2.5
Merge pull request #2089 from sn6uv/pypy_tests

Pypy tests
3783: sorting lists of edges in prufer.
3783: some additional corrections
3783: removing the new line
Merge pull request #2099 from meclark256/3783_edges

3783: sorting lists of edges in prufer.
Merge pull request #1622 from jrioux/integrals

Identify undeterminate a/0 forms of indefinite integrals