Breaking on tests_require

[cancan101]

In my setup.py, I have:

    tests_require=[
        'mock',
        'httmock',
    ]

which then leads to:

Traceback (most recent call last):
  File "/home/travis/virtualenv/python2.7.9/bin/coveralls", line 11, in <module>
    sys.exit(wear())
  File "/home/travis/virtualenv/python2.7.9/lib/python2.7/site-packages/coveralls/__init__.py", line 91, in wear
    source_files=coverage.coveralls(args.base_dir, ignore_errors=args.ignore_errors, merge_file=args.merge_file),
  File "/home/travis/virtualenv/python2.7.9/lib/python2.7/site-packages/coveralls/control.py", line 9, in coveralls
    return reporter.report(base_dir, ignore_errors=ignore_errors, merge_file=merge_file)
  File "/home/travis/virtualenv/python2.7.9/lib/python2.7/site-packages/coveralls/report.py", line 12, in report
    with open(cu.filename) as fp:
IOError: [Errno 20] Not a directory: '/home/travis/build/cancan101/project/.eggs/httmock-1.2.3-py2.7.egg/httmock.py'

It appears that since httmock is not installed, the coverage calculations get broken.