Add jumpup:bundle_install task on readme

jumpup:bundle_install always had to be explicitly declared on `INTEGRATION_TASKS` but was never documented on README.
Example: https://github.com/Helabs/pah/blob/01b028393aec307af1e17f8c66fabe2ab1c1c09a/lib/pah/files/lib/tasks/jumpup.rake#L4
Closes https://github.com/Helabs/jumpup/issues/60