Update Travis CI matrix for testing

[kevin-brown]

Fixes

Checklist

• I have made a material change to the repo (functionality, testing, spelling, grammar)
• I have read the [Contribution Guide] and my PR follows them.
• I updated my branch with the master branch.
• I have added tests that prove my fix is effective or that my feature works
• I have added necessary documentation about the functionality in the appropriate .md file
• I have added in line documentation to the code I modified

Short description of what this PR does:

• Run on Xenial
• Switch to using Tox to execute tests
• Switch to tox-travis for less repeated configuration

[codecov]

Codecov Report

Merging #750 into v4 will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##               v4     #750   +/-   ##
=======================================
  Coverage   77.26%   77.26%           
=======================================
  Files          38       38           
  Lines        1205     1205           
  Branches      172      172           
=======================================
  Hits          931      931           
  Misses        171      171           
  Partials      103      103

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
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[codecov]

Codecov Report

Merging #750 into v4 will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##               v4     #750   +/-   ##
=======================================
  Coverage   77.26%   77.26%           
=======================================
  Files          38       38           
  Lines        1205     1205           
  Branches      172      172           
=======================================
  Hits          931      931           
  Misses        171      171           
  Partials      103      103

---

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 43fc638...43b6fb9. Read the comment docs.