Update devise_ldap_authenticatable.gemspec

To work with latest ruby-net-ldap (see https://github.com/ruby-ldap/ruby-net-ldap/commit/bbef82f22494864f5c0e53d4dffb21cead3fc274)
Merge pull request #141 from tamird/patch-2

Update devise_ldap_authenticatable.gemspec
post-release version bump
Add new more general function search_by_ldap_attr, so it can query by cn when called, but the login attribute can set to email/employeeid