CAS-1253 Avoid duplicate joda-time dependencies.

Duplicate joda-time dependencies may arise in overlays that include cas-server-support-ldap. Declare direct depencency on joda-time for core and cas-server-support-ldap and exclude from transitive dependencies to ensure single consistent version.
Merge pull request #195 from serac/cas-1253

CAS-1253 Avoid duplicate joda-time dependencies.