Refactor

[ryanhiebert]

This is the first two commits from #78, since it looks like it's going to take some more effort to complete that one.

[codecov-io]

Codecov Report

Merging #79 into master will decrease coverage by 0.13%.
The diff coverage is 68.42%.

@@            Coverage Diff             @@
##           master      #79      +/-   ##
==========================================
- Coverage    75.4%   75.26%   -0.14%     
==========================================
  Files           4        5       +1     
  Lines         187      186       -1     
  Branches       46       45       -1     
==========================================
- Hits          141      140       -1     
  Misses         39       39              
  Partials        7        7

Impacted Files	Coverage Δ
src/tox_travis/after.py	57.64% <ø> (-1.91%)	⬇️
src/tox_travis/toxenv.py	94.44% <ø> (+4.08%)	⬆️
src/tox_travis/hacks.py	55.55% <55.55%> (ø)
src/tox_travis/hooks.py	86.66% <80%> (+6.66%)	⬆️

---

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 b3c99ab...45309d5. Read the comment docs.

[rpkilby]

Seems sensible to me - moves the various TRAVIS environ checks to the tox configure hook.