Python 3.7 support

[fghaas]

• Add Python 3.7 to the test matrix (and make the required related changes to .travis.yml).
• Also add Python 3.5 to the Travis configuration (an unintended omission, previously).
• Update the package's Trove classifiers to reflect supported Python versions.

[codecov-io]

Codecov Report

Merging #33 into master will not change coverage. The diff coverage is n/a.

@@           Coverage Diff           @@
##           master      #33   +/-   ##
=======================================
  Coverage   99.17%   99.17%           
=======================================
  Files           9        9           
  Lines         485      485           
=======================================
  Hits          481      481           
  Misses          4        4

---

Continue to review full report at Codecov.

Legend - Click here to learn more Δ = absolute <relative> (impact), ø = not affected, ? = missing data Powered by Codecov. Last update 8f93b5a...307714c. Read the comment docs.