remove rubygems_version

this will be set by the actual version that builds it
http://guides.rubygems.org/specification-reference/#rubygemsversion
requirement does not actually limit anything
use a string for required_ruby_version

See http://guides.rubygems.org/specification-reference/#required_ruby_version
simplify how development dependencies are declared

follows the spec reference defined in http://guides.rubygems.org/specification-reference/
Merge pull request #146 from ruby-ldap/gemspec-cleanup

Gemspec cleanup