Revert "Update e2e.sh (facebook#1167)"

This reverts commit 3a36693.

Author: gaearon

tasks/e2e.sh

@@ -56,7 +56,7 @@ root_path=$PWD
 if [ "$USE_YARN" = "yes" ]
 then
   # Install Yarn so that the test can use it to install packages.
-  npm install -g yarn@0.17.10 # TODO: remove version when https://github.com/yarnpkg/yarn/issues/2142 is fixed.
+  npm install -g yarn
   yarn cache clean
 fi