Support pip 19.0

[atugushev]

• remove process_dependency_links mention (see Remove dependency links support pypa/pip#6060)
• fix tests
• add pip 19.0 to travis and appveyor
• fix pip-compile broken with pip 19.0 #714

Changelog-friendly one-liner: Support pip 19.0

Contributor checklist

• Provided the tests for the changes
• Requested (or received) a review from another contributor
• Gave a clear one-line description in the PR (that the maintainers can add to CHANGELOG.md afterwards).

[codecov]

Codecov Report

Merging #715 into master will increase coverage by 0.03%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #715      +/-   ##
==========================================
+ Coverage   85.13%   85.17%   +0.03%     
==========================================
  Files          31       31              
  Lines        2059     2064       +5     
  Branches      308      309       +1     
==========================================
+ Hits         1753     1758       +5     
  Misses        235      235              
  Partials       71       71

Impacted Files	Coverage Δ
tests/test_cli.py	97.86% <100%> (ø)	⬆️
piptools/repositories/pypi.py	89.37% <100%> (+0.34%)	⬆️

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[vphilippon]

Left a comment about retrocompatibility, nothing too major to fix I think.

Thanks for your contribution, I've seen your activity lately, and I really appreciate it as I lack the time to properly respond or contribute.

I'll try to keep an eye on this, and get a release out as ASAP afterwards.