Only test coverage against latest stable go

[tmthrgd]

DO NOT MERGE: This is a work in progress.

The coverage Travis builds are much slower (2x on average) than the non-coverage builds. The coverage results also shouldn't diverge that significantly between go versions.

[codecov-io]

Codecov Report

Merging #775 into master will decrease coverage by 0.04%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #775      +/-   ##
==========================================
- Coverage   56.88%   56.83%   -0.05%     
==========================================
  Files          42       41       -1     
  Lines       10789    10783       -6     
==========================================
- Hits         6137     6129       -8     
- Misses       3553     3556       +3     
+ Partials     1099     1098       -1

Impacted Files	Coverage Δ
msg.go	77.63% <0%> (-0.65%)	⬇️
listen_go_not111.go

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[tmthrgd]

go: tip is the slowest build so we're only getting one minute faster with this PR. I'll revisit this once travis-ci/gimme#151 lands which should substantially speed up go: tip.