Really outdated net-ldap gem fails with Apache DS

The really outdated net-ldap gem fails with Apache DS.
See https://community.openproject.org/topics/1013.
Merge pull request #2203 from MichaelHierweck/fix/net-ldap-dependency

Really outdated net-ldap gem fails with Apache DS
Merge branch 'release/4.0' into dev

Signed-off-by: Alex Coles <alex@alexbcoles.com>

Conflicts:
	Gemfile.lock