Add vm.deprecated_library task

Find processes that are using a deprecated library and need restarting.

For example:

- Processes using a non-upgraded version of libssl:
  `vm.deprecated_library:libssl`

- Processes that are using any deleted/upgraded library:
  `vm.deprecated_library:/lib`
Merge pull request #70 from alphagov/deprecated_library

Add vm.deprecated_library task