Bug fix for pip_accel.req.Requirement.is_transitive*

* In the previous version, requirements specified in requirements files
  would be counted as transitive dependencies. This was caused by my
  misunderstanding of the pip.req.InstallRequirement.comes_from
  attribute and its possible values:

  - None means the requirement was specified on the command line

  - A pip.req.InstallRequirement object indicates a transitive
    dependency of the requirement specified as comes_from

  - A string like '-r requirements.txt (line 1)' means the
    requirement was specified in a requirements file