Issue bundle installs in driver subfolders

Correct development setup instructions so that we make sure to issue
`bundle install` in driver subfolders in addition to the top level
project. This is required to move on to run the full rspec suite.

Author: brandur

docs/development.md

@@ -4,6 +4,8 @@
 
 ```shell
 $ bundle install
+$ pushd drivers/riverqueue-activerecord && bundle install && popd
+$ pushd drivers/riverqueue-sequel && bundle install && popd
 ```
 ## Run tests