Some travis ci updates

[barambani]

This PR addresses the Coursier warning in Travis CI

Warning: a legacy coursier cache was found at /home/travis/.coursier/cache/v1 and is currently being used.

Support for that cache location will be removed in coursier 2.0.0 final, whose release is imminent.

Follow the instructions at

  https://github.com/coursier/cache-migration#cache-migration

in order to migrate your cache to the newer location.

(an example here) just changing the cache location without migrating the existing.
Also it updates the Scala Version and runs explicitly the Microsite build on 2.12.

UPDATE: I have to force Microsite make to 2.12.10. I hope it's ok while waiting for the docs port to alternatives (paradox + mdoc ?).

[codecov-commenter]

Codecov Report

Merging #3471 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #3471   +/-   ##
=======================================
  Coverage   92.05%   92.05%           
=======================================
  Files         382      382           
  Lines        8330     8330           
  Branches      228      213   -15     
=======================================
  Hits         7668     7668           
  Misses        662      662           

[travisbrown]

I would have guessed that this would still run makeMicrosite for every Scala version, but that doesn't seem to be the case, so 👍 from me.