Necessary to get the application up and running locally

Was encountering a problem with the bundler version
Used the solution here:
https://stackoverflow.com/questions/47026174/find-spec-for-exe-cant-find-gem-bundler-0-a-gemgemnotfoundexception

Signed-off-by: Chris Croome <chris@webarchitects.co.uk>

docker-compose.yml

@@ -17,6 +17,7 @@ services:
       BUNDLE_PATH: /vendor
     command: >
       sh -c '
+        gem update --system &&
         bundle &&
         bundle exec jekyll build --destination /site --watch
       '