Missing license information causes error messag when intalling

[gene1wood]

When installing proxifier this error message is shown

       re-installing proxifier...
       WARNING:  licenses is empty, but is recommended.  Use a license identifier from
       http://spdx.org/licenses or 'Nonstandard' for a nonstandard license.
       WARNING:  See https://guides.rubygems.org/specification-reference/ for help
         Successfully built RubyGem
         Name: proxifier
         Version: 1.0.3
         File: proxifier-1.0.3.gem
       Successfully installed proxifier-1.0.3

PR #13 fixes this problem.