get rid of vestal_versions!

I implemented my own version of the latest_changes method that should replace
the heavyweight vestal_versions gem which affected our database.

The replacement consists of finding, and listing, the differences between two
hashes. The attributes of the original package is obtained via
`orig_package.attributes`, and the attributes of the updated package is obtained
via `package.attributes`. Comparing the differences between those two will give
us a replacement for `latest_changes`.

The comparison difference in hash is performed by the `diff_custom` method added
to the Hash class (via open classes). The current `diff` method provided by Hash
did not list the old and the new values, but only the new values. The
`diff_custom` method will list both the old and new values in an array.

The `diff_custom` method is defined inside `environment.rb`.