Require ruby-net-ldap 0.5.0 to avoid encoding issues

Note this pull request breaks normal installs as it requires ruby-net-ldap v0.5.0 or later which has not been pushed to rubygems.org.

There's an issue open about this and I e-mailed the author in the hopes they can put the already tagged 0.5.0 in rubygems.org.

In particular, the encoding issues at hand prevent search by GUID from working properly. Querying and saving the GUID is important to avoid problems where administrators may rename containers, etc. Querying by name is not reliable in those cases.