Unpin the default Yarn version from 1.9.x (#569)

Now that 1.11.0 is released with https://github.com/yarnpkg/yarn/pull/6413 merged,
Heroku users should see fewer failed builds due to 500 responses from
the registry.

lib/binaries.sh

 install_yarn() {
   local dir="$1"
-  local version=${2:-1.9.x}
+  local version=${2:-1.x}
   local number
   local url