Explicitly specify to use go v1.9.x for Travis CI. Fixes #752

[kamphaus]

Fixes #752

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@@            Coverage Diff            @@
##             master     #753   +/-   ##
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  Coverage          ?   80.79%           
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  Files             ?      166           
  Lines             ?     9478           
  Branches          ?        0           
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  Hits              ?     7658           
  Misses            ?     1388           
  Partials          ?      432

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[elliotchance]

Reviewed 1 of 1 files at r1.
Review status: complete! all files reviewed, all discussions resolved

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.travis.yml, line 4 at r1 (raw file):

go:
  - "1.9.x"

Wow, I can't believe I never actually specified a Go version in CI... that's my bad :|

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