Create a Scala 2.11 branch

[kailuowang]

Strictly speaking the scala_2.11 branch is already created. This PR strips out Scala 2.12+ related code on this branch.

[codecov-io]

Codecov Report

Merging #3052 into scala_2.11 will decrease coverage by 0.09%.
The diff coverage is n/a.

@@              Coverage Diff              @@
##           scala_2.11    #3052     +/-   ##
=============================================
- Coverage       93.46%   93.37%   -0.1%     
=============================================
  Files             368      382     +14     
  Lines            6979     7155    +176     
  Branches          184      195     +11     
=============================================
+ Hits             6523     6681    +158     
- Misses            456      474     +18

Impacted Files	Coverage Δ
...ain/scala/cats/kernel/compat/TraversableOnce.scala	0% <ø> (ø)
core/src/main/scala/cats/instances/stream.scala	97.22% <ø> (ø)
...tances/ScalaVersionSpecificParallelInstances.scala	90.9% <ø> (ø)
.../src/main/scala/cats/data/OneAndLowPriority4.scala	12.5% <ø> (ø)
...cala/cats/kernel/compat/scalaVersionSpecific.scala	100% <ø> (ø)
...ala/cats/kernel/compat/WrappedMutableMapBase.scala	0% <ø> (ø)
core/src/main/scala/cats/compat/SortedSet.scala	100% <ø> (ø)
...scala/alleycats/compat/IterableEmptyInstance.scala	0% <ø> (ø)
.../scala/cats/data/ScalaVersionSpecificPackage.scala	50% <ø> (ø)
.../cats/kernel/compat/scalaVersionMoreSpecific.scala	100% <ø> (ø)
... and 31 more

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[travisbrown]

I spent a few minutes going over this the other day, and it seemed fine to me, although I didn't look very closely. The tests pass, so I'd say ship it—it belongs to the unfortunate folks who still have to care about 2.11 now. 😄

[kailuowang]

@LukaJCB, got a minute to review the changes (mainly to the build file)?

[LukaJCB]

Looks good :)