Fix build error with ruby 1.9.3p545

Fix for this error during bundle exec gem build omniauth-uaa-oauth2.gemspec with ruby 1.9.3p545
ERROR:  While executing gem ... (Gem::InvalidSpecificationException)
    duplicate dependency on cf-uaa-lib (< 2.0, >= 1.3.1), (< 2.0, >= 1.3.1) use:
    add_runtime_dependency 'cf-uaa-lib', '< 2.0, >= 1.3.1', '< 2.0, >= 1.3.1'