update license information

Thank you very much for your work on innertube! I'm working on packaging this gem for Fedora, and the Fedora Packaging Guidelines suggest that I ask you to include the full text of the ASL 2.0 license in your project. By including a copy of the exact license text, you will make the terms of the innertube license very clear.

Also, in the specific case of innertube's ASL 2.0 license, all derivative copies of innertube are required to contain the full text of the ASL 2.0 license (as described in the sentence "You must give any other recipients of the Work or Derivative Works a copy of this License") So in order to ease development and distribution of innertube, it makes sense to simply include the full text in the upstream project.

More details about this can be found on Fedora's wiki, under the "License Text" section: https://fedoraproject.org/wiki/Packaging:LicensingGuidelines#License_Text

The first commit in this pull request adds the full license text to the LICENSE file, and the second commit adds the appropriate metadata to innertube's gemspec.