Only do branch builds on master.

[Lukasa]

Because the Travis build time is quite long, it'd be much better to reduce the number of builds we have to do. Normally, from branches within the repo, Travis will build them twice: once for the branch, once for the PR. We shouldn't do that: one build is plenty.

[codecov-io]

Codecov Report

Merging #15 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master      #15   +/-   ##
=======================================
  Coverage   81.96%   81.96%           
=======================================
  Files           5        5           
  Lines         693      693           
  Branches      163      163           
=======================================
  Hits          568      568           
  Misses         76       76           
  Partials       49       49

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[sethmlarson]

LGTM

[njsmith]

If this becomes a problem due to like, blocking Travis builds on other projects in the organization (that's a thing, right?), then it can almost certainly be trimmed further without too much pain. Like we could probably drop 3.4 without anyone noticing, and that's 1/3 of the builds right there. And we probably don't need to re-run the whole test suite twice with and without wsaccel on every platform.

[Lukasa]

Indeed. Right now this was just a trivial win of removing duplicate testing.