Use circleci ramdisk (#16)

https://circleci.com/docs/2.0/executor-types/#ram-disks

Author: marcqualie

Diff for: .circleci/config.yml

@@ -4,6 +4,7 @@ jobs:
   build:
     docker:
       - image: circleci/ruby:2.6.6
+    working_directory: /mnt/ramdisk
     steps:
       - checkout
       - run: bundle install