.travis.yml: add 1.8.x builds to travis

[davecheney]

No description provided.

[codecov-io]

Codecov Report

Merging #691 into master will increase coverage by 0.16%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #691      +/-   ##
==========================================
+ Coverage   52.97%   53.13%   +0.16%     
==========================================
  Files          42       40       -2     
  Lines        3160     3045     -115     
==========================================
- Hits         1674     1618      -56     
+ Misses       1342     1249      -93     
- Partials      144      178      +34

Impacted Files	Coverage Δ
build.go	75% <ø> (-9.91%)	❌
cmd/gb/build.go
internal/depfile/depfile.go
depfile.go	32.06% <ø> (+24.06%)	✅

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[davecheney]

Test flake caused by gopkg.in. Updates #690