Start a Scala 2.11 dedicated branching scheme:

• Branch master - drops Scala 2.11, maintains is BC with 1.x
• Branch scala2_11 - Scala 2.11 code
  New changes to master have to be ported to branch scala2_11 in a 2.11 BC compatible way and try to maintain as use site source compatible as possible - e.g. a new type class method added to master is added to a syntax trait method in scala2_11 branch.

Release Cats 2.1 RC1 on both Scala 2.11 and Scala 2.12+
Note that 2.11 and 2.12+ code are NOT source compatible with each other, however, they will still maintain source compatibility with 2.0 and 1.x.