Install the specified version of Yarn

CircleCI has a default version of Yarn, but the version cannot be
specified. This is why we have to install our own version of Yarn that
matches the one in package.json. This prevents yarn failing with a
mismatched version error.

circle.yml

@@ -4,13 +4,23 @@ machine:
   node:
     version: 6.11.5
   environment:
-    PATH: "${PATH}:${HOME}/${CIRCLE_PROJECT_REPONAME}/node_modules/.bin"
+    YARN_VERSION: 1.2.1
+    PATH: "${PATH}:${HOME}/.yarn/bin:${HOME}/${CIRCLE_PROJECT_REPONAME}/node_modules/.bin"
 
 dependencies:
+  pre:
+    - |
+      if [[ ! -e ~/.yarn/bin/yarn || $(yarn --version) != "${YARN_VERSION}" ]]; then
+        echo "Downloading and installing Yarn"
+        curl -o- -L https://yarnpkg.com/install.sh | bash -s -- --version $YARN_VERSION
+      else
+        echo "The correct version of Yarn is already installed"
+      fi
   override:
     - yarn
   cache_directories:
     - ~/.cache/yarn
+    - ~/.yarn
 
 test:
   pre: