Attempt another fix for yarn in CircleCi (#357)

Adapting stuff from https://circleci.com/docs/1.0/yarn.

circle.yml

@@ -17,21 +17,23 @@ machine:
       -XX:+UseConcMarkSweepGC
       -XX:+CMSClassUnloadingEnabled
       -Xverify:none
+    PATH: "${HOME}/.yarn/bin:${HOME}/.config/yarn/global/node_modules/.bin:${PATH}"
 
 dependencies:
   cache_directories:
     - "tmp"
     - "~/.boot/cache/lib"
     - "~/.boot/cache/bin"
     - "~/.yarn"
+    - "~/.cache/yarn"
     - "~/.config/yarn"
     - "/usr/local/bin/boot"
     - "/usr/local/lib/node_modules"
     - "/usr/local/Cellar"
   pre:
     - curl -o- https://raw.githubusercontent.com/creationix/nvm/v0.33.8/install.sh | bash && export NVM_DIR="$HOME/.nvm" && [ -s "$NVM_DIR/nvm.sh" ] && \. "$NVM_DIR/nvm.sh"
     - nvm install v9.2.0
-    - if [[ ! -d "$HOME/.yarn" ]]; then curl -o- -L https://yarnpkg.com/install.sh | bash; else export PATH="$HOME/.yarn/bin:$HOME/.config/yarn/global/node_modules/.bin:$PATH"; fi
+    - if [[ ! -d "$HOME/.yarn" ]]; then curl -o- -L https://yarnpkg.com/install.sh | bash; fi
     - |
       if [[ ! -e /usr/local/bin/boot ]];
       then