chore: avoid double travis build for PRs (ipfs#1133)

Disable building branches that aren't master - builds will happen
for all PRs anyway, so this change means we don't get a branch
build and a PR build on each PR.

.travis.yml

@@ -1,6 +1,11 @@
 language: node_js
 cache: npm
 
+branches:
+  only:
+  - master
+  - /^release\/.*$/
+
 stages:
   - check
   - test