Deprecate Python 3.4 and add support and testing for Python 3.8

[tleonhardt]

Removed extra dependencies and imports required by Python 3.4 as well as unit testing on Python 3.4 from all platform.s

Added support for Python 3.8 in setup.py and unit testing for python 3.8beta on Travis CI.

Now that we support Python 3.5+, this opens up the possibility for beginning to use libraries like pathlib.

Closes #661
Closes #693

TODO:

• Updated CHANGELOG

[codecov]

Codecov Report

Merging #700 into master will decrease coverage by 0.02%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #700      +/-   ##
==========================================
- Coverage   95.47%   95.45%   -0.03%     
==========================================
  Files          11       11              
  Lines        3251     3255       +4     
==========================================
+ Hits         3104     3107       +3     
- Misses        147      148       +1

Impacted Files	Coverage Δ
cmd2/cmd2.py	95.58% <100%> (-0.06%)	⬇️
cmd2/pyscript_bridge.py	100% <100%> (ø)	⬆️
cmd2/parsing.py	100% <0%> (ø)	⬆️
cmd2/plugin.py	100% <0%> (ø)	⬆️
cmd2/history.py	100% <0%> (ø)	⬆️

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