update license to Apache 2.0

[eirinikos]

This commit replaces the MIT license with the Apache 2.0 license.

(All CNCF projects should use the Apache License 2.0 - please see opentracing/specification#61 for context.)

Note:

• The copyright reference to OT is as such: Copyright The OpenTracing Authors

We would like to give previous contributors a 7-day period to air any objections. If you have any concerns or feedback, please notify us by April 4th. Thanks!

(CC @dankohn @bhs)
(CC previous contributors: @leslie-lei @supermarin)

[eirinikos]

@bhs 👋

For whatever reason, Licensee is still not identifying this repo's license type as Apache 2.0. (Licensee being the tool that GitHub uses to tag repositories with their license types.)

Could we hold off on merging this PR until I push a new commit to fix the problem? I hope to have a solution in the next day or so.

(Right now, this problem affects OT-Go, OT-Objc, OT-PHP, and OT-Ruby.)

[bhs]

@eirinikos

Could we hold off on merging this PR until I push a new commit to fix the problem?

Sure thing. Just let me know when the code is ready.

[eirinikos]

For some reason, in some situations, using a # License header in a repo's README will cause Licensee to scan the README as a license file.

Replacing # License with ## Licensing allows us to bypass this problem.

If there are no objections to switching to Apache 2.0, then this PR is ready for a merge on Wednesday, 4/5!