Update dependencies.

[vejeta]

@larsrh, @imarios, @OlivierBlanvillain:

• Update scalatest imports removing deprecated ones
• Update some links in the docs
• Moves to sbt 1.3.4 from 0.13.18

Note: There is some influx of changes on scalatest 3.1 that should be stabilized on scalatest 3.2, so holding until then. More info in: scalatest/scalatest#1735

Note 2: We did not migrate from tut to mdoc due to this issue: scalameta/mdoc#248

[codecov-io]

Codecov Report

Merging #407 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #407   +/-   ##
=======================================
  Coverage   96.83%   96.83%           
=======================================
  Files          60       60           
  Lines        1044     1044           
  Branches        3        3           
=======================================
  Hits         1011     1011           
  Misses         33       33

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[larsrh]

Thanks!