Fix "Apache License, Version 2.0" spelling

[don-vip]

There are many Java libraries licensed under "Apache License, Version 2.0" that do not use its official spelling. This causes issues like https://issues.apache.org/jira/browse/MPIR-382: with every library defining its own spelling, it's difficult in large projects to have a clear view of all licenses in use. This PR changes the license spelling to the official one, as advised by Maven developers.

[lsimons]

Nice one. Cherry-picked into #16 !