Fixes sbt Plugin Publish

[juanpedromoreno]

What this do?

Sbt plugins can only be compiled with Scala 2.12.10. This PR fixes it in the Travis CI pipeline.

Checklist

• Reviewed the diff to look for typos, println and format errors.
• Updated the docs accordingly.

[codecov]

Codecov Report

Merging #818 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #818   +/-   ##
=======================================
  Coverage   71.63%   71.63%           
=======================================
  Files          69       69           
  Lines        1033     1033           
  Branches       19       19           
=======================================
  Hits          740      740           
  Misses        293      293

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