LICENSE: Migrate from MIT to Apache 2.0 license

Adopt Apache 2.0 license in order to have uniform licenses across Quick
and Nimble.

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I understand that changing a software license can sometimes be a controversial topic, but as more and more organizations begin to use Quick and Nimble, I'd like to have cohesive licensing across the two projects. Nimble currently uses Apache 2.0. From what I understand, Apache 2.0 is considered to be just as permissive as MIT, with some additional trademark protections for the Quick project.

I'd like to gauge response to this change; please weigh in if you have any thoughts. Absent of any objections, I plan on merging this change by November 1st.

/cc @Quick/contributors @alexbasson @garnett @paulyoung @dmeehan1968 @jspahrsummers @nerdyc @orta