Cache yarn on Travis CI.

RubenVerborgh · RubenVerborgh · commit 09bd49ebc130 · 2017-08-21T10:29:03.000-04:00
diff --git a/.travis.yml b/.travis.yml
@@ -5,3 +5,4 @@ script:
   - yarn test
 after_success:
   - yarn coverage:report
+cache: yarn
