Port to 2.13

[d6y]

I've also dropped 2.11 and 2.12 support.

[codecov-io]

Codecov Report

Merging #11 into develop will decrease coverage by 0.47%.
The diff coverage is 50.00%.

@@             Coverage Diff             @@
##           develop      #11      +/-   ##
===========================================
- Coverage    73.11%   72.64%   -0.48%     
===========================================
  Files           19       19              
  Lines          212      212              
  Branches         8        4       -4     
===========================================
- Hits           155      154       -1     
- Misses          57       58       +1     

Impacted Files	Coverage Δ
src/main/scala/csvside/Mighty.scala	57.89% <16.66%> (-5.27%)	⬇️
src/main/scala/csvside/Read.scala	78.26% <100.00%> (ø)

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