Test with Python 3.7

[GPHemsley]

• Remove version numbers from requirements file so we always use the latest.
  • The version of lxml we had pegged doesn't work with Python 3.7.
• Update the Travis CI config to fix testing on Python 3.7.
  • Also remove the duplicate 3.8-dev, which is the same as nightly (for now).

[codecov-io]

Codecov Report

Merging #33 into master will decrease coverage by 32.8%. The diff coverage is n/a.

@@             Coverage Diff             @@
##           master      #33       +/-   ##
===========================================
- Coverage   99.83%   67.02%   -32.81%     
===========================================
  Files           7        7               
  Lines         617      919      +302     
===========================================
  Hits          616      616               
- Misses          1      303      +302

Impacted Files	Coverage Δ
setup.py	24.13% <0%> (-63.37%)	:arrow_down:
tests/test_parser.py	56.69% <0%> (-43.31%)	:arrow_down:
tests/test_jurisdiction.py	65.98% <0%> (-34.02%)	:arrow_down:
clarify/parser.py	70.03% <0%> (-29.97%)	:arrow_down:
clarify/jurisdiction.py	90.54% <0%> (-9.46%)	:arrow_down:

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[GPHemsley]

Oh, it seems this breaks code coverage. I'll have to look into this further.