fix nil tls_options
encryption defaults empty :tls_options
update mock
Merge pull request #176 from ruby-ldap/fix-nil-tls-options

Fix nil :tls_options
Add rubocop to project

This adds https://github.com/bbatsov/rubocop to the project with an auto
generated `.rubocop_todo.yml`. The violations can be addressed over time without
breaking the build.