Cache yarn on Travis CI.

RubenVerborgh · RubenVerborgh · commit 5faf85ede0f5 · 2017-08-15T01:03:52.000-04:00
diff --git a/.travis.yml b/.travis.yml
@@ -6,3 +6,4 @@ script:
   - yarn test
 after_success:
   - yarn coverage:report
+cache: yarn
