Verify ssl certificates by default, and add disable option

• Cherry picked a few specific commits from intridea/omniauth-ldap instead of merging master to avoid bringing in a mess of dependency bump commits
• Changed the way we specify dependencies http://yehudakatz.com/2010/12/16/clarifying-the-roles-of-the-gemspec-and-gemfile
• Added SSL certificate hostname verification by bumping the net-ldap gem
• Added SSL certificate verification by setting tls_options appropriately when instantiating Net::LDAP
• Added ca_file and ssl_version options (ported from !3 (closed)). This will be especially helpful for some customers after we enable certificate verification by default.

This is the first step for https://gitlab.com/gitlab-org/gitlab-ce/issues/30420.

• Close MR !3 (closed) after this is merged