Travis changes

[javdrher]

• Enables caching, however saves less speed than I was hoping for
• Added a directive which will be required in a forthcoming travis update
• Travis configuration now installs optional dependencies defined in setup.py, rather than listing requirements again.

[codecov-io]

Codecov Report

Merging #30 into master will not change coverage. The diff coverage is n/a.

@@           Coverage Diff           @@
##           master      #30   +/-   ##
=======================================
  Coverage   99.62%   99.62%           
=======================================
  Files           7        7           
  Lines         537      537           
=======================================
  Hits          535      535           
  Misses          2        2

---

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 336cada...5cc8d2d. Read the comment docs.