Refactor CI

[partmor]

• Update tox.ini
• Drop py35 support
• Fix OSX issue on travis-ci

[codecov-io]

Codecov Report

Merging #12 into master will not change coverage.
The diff coverage is 52%.

@@           Coverage Diff           @@
##           master      #12   +/-   ##
=======================================
  Coverage   72.42%   72.42%           
=======================================
  Files           2        2           
  Lines         214      214           
  Branches       12       12           
=======================================
  Hits          155      155           
  Misses         59       59

Impacted Files	Coverage Δ
src/ezaero/vlm/plotting.py	0% <0%> (ø)	⬆️
src/ezaero/vlm/steady.py	100% <100%> (ø)	⬆️

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