Allow [] default for Mdm:WebVuln#params

[#44034071]

Mdm::WebVuln#params was supposed to have a default of [], but its
serialzier had a hard-coded default of {}.  Support was added to the
serializer to allow different defaults and in-memory params was
normalized to [] if set or read as nil.
Require factory_girl 4.1.0 which supports unqualified generate

[#44034071]

The ability to call `generate` instead of `FactoyGirl.generate` in
factory definitions was added in 4.1.0.  Since the Gemfile.lock is not
committed (since this is a gem) use a version restriction in the Gemfile
to ensure at least 4.1.0 is installed.
Merge pull request #10 from rapid7/bug/web-vuln-blank-params

Allow [] default for Mdm:WebVuln#params