chore(travis): use yarn 1.5.1

Author: christopherthielen

.travis.yml

@@ -1,12 +1,16 @@
 language: node_js
 node_js: 8.9.1
-before_install: sh -e /etc/init.d/xvfb start
+before_install:
+- sh -e /etc/init.d/xvfb start
+- export PATH=$HOME/.yarn/bin:$PATH
+- curl -o- -L https://yarnpkg.com/install.sh | bash -s -- --version 1.5.1
 script:
 - if [ "x${CORE_BRANCH}x" != "xx" ] ; then ./node_modules/.bin/publish_yalc_package
   .downstream_cache/core https://github.com/ui-router/core.git && yalc add @uirouter/core
   ; fi
 - tsc
 - yarn test
+- yarn docs
 - yarn test:downstream
 env:
   global: