Assorted CI improvements, including adding flake8 linting

[nth10sd]

• Switches to use Ubuntu 14.04 Trusty in Travis CI
• Updates pip, setuptools and virtualenv prior to installing other Python libraries
• Adds support for flake8 linting
  • Fixes issues identified by flake8
• Adds support for pylint, disabled for now.
  • Will probably be enabled with fixes via another PR

The tests show red prior to the flake8 fixes landing, they show green thereafter. I have tested on another branch that a deliberate flake8 failure will correctly turn Travis and AppVeyor red.

[codecov-io]

Codecov Report

Merging #22 into master will not change coverage. The diff coverage is 57.14%.

@@           Coverage Diff           @@
##           master      #22   +/-   ##
=======================================
  Coverage   84.31%   84.31%           
=======================================
  Files           7        7           
  Lines        1211     1211           
=======================================
  Hits         1021     1021           
  Misses        190      190

Impacted Files	Coverage Δ
lithium/examples/arithmetic/product_divides.py	100% <ø> (ø)	:arrow_up:
interestingness/ximport.py	68.18% <ø> (ø)	:arrow_up:
setup.py	0% <0%> (ø)	:arrow_up:
__init__.py	100% <100%> (ø)	:arrow_up:
lithium/tests.py	95.07% <100%> (ø)	:arrow_up:
lithium/lithium.py	79.74% <48.48%> (ø)	:arrow_up:

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Continue to review full report at Codecov.

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[nth10sd]

The switch to 14.04 and updating of pip et al. ensures using latest Python libraries without being on an ancient OS.

[nth10sd]

The only repo-wide rule for flake8 for now is the one ignoring lines that are too long.

[jschwartzentruber]

Thanks!