ci: enable travis npm cache (#110)

* ci: enable travis npm cache

* ci: upgrade travis version

Thanks @zerdos

* ci: switch to using yarn cache

Author: nhooyr

.travis.yml

@@ -1,6 +1,6 @@
 language: node_js
 node_js:
-- 8.10.0
+- 8.15.0
 env:
   - VERSION="1.32.0-$TRAVIS_BUILD_NUMBER"
 matrix:
@@ -35,3 +35,4 @@ deploy:
   on:
     repo: codercom/code-server
     branch: master
+cache: yarn