Pip 18 support

[suutari]

Pip 18 has been released and there is some internal API changes which have to addressed to make the resolver work again.

Already did this for Prequ, and it shouldn't be very hard to port the changes to pip-tools. Here's the commit that adds Pip 18 support to Prequ: suutari@fdc6f1d#diff-395e820748909fbb1b029281c5bbdf8c

[rpkilby]

For anyone else surprised by the pip version, note that pip 18 is now using a calendar based versioning and release scheme.

[vphilippon]

Thanks for the report @suutari, and thanks for the info @rpkilby .

As you might have noted, I don't have much time to give to pip-tools right now. If someone wants to submit the PR, I'll do my best to join in on the review, as well as take care of the release in due time.