Fix warnings in gemspec

Why is this change being made?

This MR fixes warnings that appear when building the gem:

$ gem build gitlab-security_report_schemas.gemspec 
WARNING:  licenses is empty, but is recommended.  Use a license identifier from
http://spdx.org/licenses or 'Nonstandard' for a nonstandard license.
WARNING:  open-ended dependency on activesupport (>= 5) is not recommended
  if activesupport is semantically versioned, use:
    add_runtime_dependency 'activesupport', '~> 5'
WARNING:  See https://guides.rubygems.org/specification-reference/ for help
  Successfully built RubyGem
  Name: gitlab-security_report_schemas
  Version: 0.1.0.min0.0.0.max0.0.0
  File: gitlab-security_report_schemas-0.1.0.min0.0.0.max0.0.0.gem

Additionally, spec.homepage had an old version of the project name.