Backports #3133 to scala_2.11

[jackcviers]

• Adds scala version specific src directories
• Backports PR Fix NonEmptyChain.fromSeq signature on 2.12 #3133 to add scala 2.11 support

[codecov-io]

Codecov Report

Merging #3134 into scala_2.11 will not change coverage.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           scala_2.11   #3134   +/-   ##
==========================================
  Coverage        93.4%   93.4%           
==========================================
  Files             382     382           
  Lines            7155    7155           
  Branches          195     194    -1     
==========================================
  Hits             6683    6683           
  Misses            472     472

Impacted Files	Coverage Δ
core/src/main/scala/cats/data/NonEmptyChain.scala	85.39% <ø> (ø)	⬆️
core/src/main/scala/cats/data/NonEmptyList.scala	99.33% <100%> (ø)	⬆️
core/src/main/scala/cats/instances/option.scala	100% <100%> (ø)	⬆️
core/src/main/scala/cats/data/NonEmptyVector.scala	99.13% <100%> (ø)	⬆️
.../src/main/scala/cats/laws/discipline/MiniInt.scala	97.29% <100%> (ø)	⬆️

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[travisbrown]

This looks reasonable to me, but I think we should wait for #3133 to be merged. It would also probably be easier to backport features in the order they were added to post-2.0.0 master, but I don't think that should necessarily block this PR. (Also, relatedly, I think it'd be better to separate the Scalafmt update commit from the other part.)

[travisbrown]

Wait, this doesn't need Scala-version-specific code because the scala_2.11 branch will only ever be used to publish for 2.11.

[jackcviers]

Sure, I can remove the 2.11 branch and seperate out the build parts. It wasn't passing the scalafmt build step on travis, so I added it in.

[jackcviers]

Also, I put in the 2.11, 2.12, and 2.13+ source directories in so that if you ever did want to merge the 2.11 back into master, that would be possible without a great deal of trouble.

I have to wonder, why not simply have a 2.11 source directory on master where all code changes go for 2.11, rather than a full branch? The way it is, you will have to maintain tags for two branches going forward.

[travisbrown]

@jackcviers That would involve a lot of code duplication, and the plan is to drop 2.11 entirely very soon.

[larsrh]

I'm closing all 2.11-related tickets since there is not enough community interest in further releases.