Add license information to the gemspec

[reiz]

Tool like VersionEye are using this information for license compliance analysis.

Before submitting a PR that modifies geographical data, please read the Wiki on how to do this properly. This will save everyone a lot of time, and get your PR merged quicker!

For non-trivial PRs, please add an entry to the CHANGELOG (following the convention in place).

If the changes are geography related (overlay/locale modifications) place the entry under "Geographic Modifications".

Please follow this format exactly:

[#<pr-number>] <PR description _without_ an ending period> ([@<github-handle>](<github-profile>))

Please replace the text above with a summary of your changes

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Before submitting the PR please make sure that you have done the following:

• I have read the Wiki on how to make geographic changes if this PR modifies geographic data
• I have added a CHANGELOG entry if appropriate