Update versapay.gemspec

Get rid of warning:
versapay at /vagrant/src/payroll/vendor/gems/ruby/1.9.1/bundler/gems/versapay-45d0d88755e6 did not have a valid gemspec.
This prevents bundler from installing bins or native extensions, but that may not affect its functionality.
The validation message from Rubygems was:
  duplicate dependency on activesupport (= 2.3.11, development), (>= 3.1.0) use:
    add_runtime_dependency 'activesupport', '= 2.3.11', '>= 3.1.0'