Release 1.2.0.

Change-Id: I7ad65698ac3ecc45b1f7469ca87d7da0bdfc8fdb
Opening 1.2.1 for development.

Change-Id: I15e818502cd937d509dc73578b206183ee68f749
Express our required unittest2 version.

This should avoid incompatibilities as reported in
https://github.com/testing-cabal/testtools/issues/121.

Change-Id: I319e2e572c40a589ecbb2be74247f8b1ee7f192b