1.0: Wrapping pip list --outdated

Pip-review dropped its own update-checking logic and now relies on pip list --outdated instead. This improves the consistency with pip and solves many old issues.

• breaking Support for Python 2.6 and Python 3.2 has been dropped entirely. Only Python versions 2.7 and 3.3+ are now supported.
• breaking pip-review is now guaranteed to list the same updates as pip list --outdated. While the reliance on pip list makes pip-review more reliable overall, it does introduce some subtle changes in behaviour. pip-review may find updates that it couldn't detect before. pip-review will also copy errors in the available updates from pip list if those occur. For example, version 7.0 of pip used to include prerelease updates even if the --pre flag wasn't passed. If you are using pip-review in an automated workflow which relies on very particular update detection, you may wish to compare the new behaviour of pip-review with version 0.5.3. #18 #21 #32 #41 #52 #53 #55
• breaking pip-review does not accept the --editables option anymore. Instead, it forwards the --editable option to pip list. Note the disappearance of the trailing 's' in the option name. #32 #53
• Unrecognized options are forwarded unchanged to pip list. As a consequence, you now have much more control over the way in which packages are checked for updates. #6 #21 #27 #32 #53
• The Travis CI build status now accurately reflects the development status in Python versions 2.7, 3.3 and 3.6. #15 #42 #48 #51
• pip-review still support all versions of pip from 1.5.6 onwards. In pip version 9.0 and later, pip list --outdated is called with the additional flag --format=json for maximal reliability. In older versions of pip, a simple but reliable custom parser is used to extract the required information from the text-based output. #32 #41 #53
• Internal calls to pip now universally process the command as a list rather than as a concatenated string. This makes the subprocess behaviour more reliable. #32 #44 #45 #50 #53 #55
• pip-review can now again update itself even if invoked as pip-review. In version 0.5 through 0.5.3, this was only possible if invoked as python -m pip_review. #44 #45 #47

Thanks to @xoviat and @bil-elmoussaoui for making important contributions to this release.