Run travis CI only on master branch and PRs (#114)

If a PR is created using a branch pushed to origin, this triggers travis
CI twice, once for the PR and another for the branch. So run travis CI for
only the master branch and when PRs are created using origin branches.
If CI needs to be run on any specific branch other than master, it would
need to be whitelisted.

Co-authored-by: Mat Ryer <matryer@users.noreply.github.com>

.travis.yml

@@ -2,6 +2,10 @@ language: go
 
 sudo: false
 
+branches:
+  only:
+    - master
+
 go:
   - 1.11.x
   - 1.12.x