Improve travis testing

[jonc125]

• Check with both pinned packages and setup.py install
• Use consistent flake8 & isort checks
• Add code coverage

Fixes #41, fixes #42.

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (develop@b54edd5). Click here to learn what that means.
The diff coverage is 100%.

@@            Coverage Diff             @@
##             develop      #66   +/-   ##
==========================================
  Coverage           ?   87.29%           
==========================================
  Files              ?        8           
  Lines              ?      842           
  Branches           ?      181           
==========================================
  Hits               ?      735           
  Misses             ?       70           
  Partials           ?       37

Impacted Files	Coverage Δ
cellmlmanip/units.py	81.65% <ø> (ø)
cellmlmanip/__init__.py	100% <100%> (ø)
cellmlmanip/mathml2sympy/__init__.py	100% <100%> (ø)

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[MichaelClerx]

Couple of comments / questions!