Update Scala 2.13 version to M4

[fthomas]

No description provided.

[fthomas]

This needs to wait until dependencies are available for 2.13.0-M4:

[warn] 	::::::::::::::::::::::::::::::::::::::::::::::
[warn] 	::          UNRESOLVED DEPENDENCIES         ::
[warn] 	::::::::::::::::::::::::::::::::::::::::::::::
[warn] 	:: com.chuusai#shapeless_2.13.0-M4;2.3.3: not found
[warn] 	:: org.scalacheck#scalacheck_2.13.0-M4;1.14.0: not found
[warn] 	::::::::::::::::::::::::::::::::::::::::::::::

• Publish for 2.13.0-M4 milessabin/shapeless#832
• publish for Scala 2.13.0-M4? typelevel/scalacheck#410

[codecov]

Codecov Report

Merging #512 into master will decrease coverage by 1.23%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #512      +/-   ##
==========================================
- Coverage   94.26%   93.03%   -1.24%     
==========================================
  Files          62       62              
  Lines         680      689       +9     
  Branches       14       14              
==========================================
  Hits          641      641              
- Misses         39       48       +9

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