Support go1.13

[gmlewis]

Add support for go1.13 and bump Travis CI tests from 1.11 to 1.12 and 1.13.

[codecov]

Codecov Report

Merging #1279 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1279   +/-   ##
=======================================
  Coverage   73.42%   73.42%           
=======================================
  Files          86       86           
  Lines        6040     6040           
=======================================
  Hits         4435     4435           
  Misses        836      836           
  Partials      769      769

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[willnorris]

You actually don't need to add 1.13 explicitly... the "1.x" version will always test the latest. So what I've been doing is test master plus the latest two version, with "1.x" always capturing the most recent version. So this should really now just be:

go:
 - "1.x"
 - "1.12.x"
 - "master"

I'd rather not go beyond these three, primarily to keep our build times down.

[gmlewis]

Thank you, @willnorris!
I'll delete that line.