Add python 3.7 for travis

[r9y9]

No description provided.

[codecov-io]

Codecov Report

Merging #84 into master will decrease coverage by 0.11%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master      #84      +/-   ##
==========================================
- Coverage   83.85%   83.73%   -0.12%     
==========================================
  Files          31       31              
  Lines        1697     1697              
==========================================
- Hits         1423     1421       -2     
- Misses        274      276       +2

Impacted Files	Coverage Δ
nnmnkwii/io/hts.py	94.47% <0%> (-0.62%)	⬇️
nnmnkwii/frontend/merlin.py	77.55% <0%> (-0.35%)	⬇️

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[r9y9]

Need to wait until MattShannon/bandmat#10 is fixed.