Update checkers extension for Scalatest

[lloydmeta]

As of ScalaTest 3.1.0, org.scalatest.prop.Checkers has been
deprecated. Moving forward only org.scalatest.check.Checkers will
be supported.

Reference: Scalatest code

[frgomes]

Please have a look at #84 and #85.
The idea is that sbt-doctest should not have intrinsic dependencies from any test framework and should not suppose (or impose) that the user employs a specific version of a given test framework.
Maybe we could work together on this task. I was considering refactor the code somehow so that the user chooses the desired test framework and, from that point, all details are defined in order to comply to such decision. More on that at #88.

[lloydmeta]

That's a good point; this shouldn't be hardcoded according to version and framework. I'm in favour of doing whatever is simplest, as long as there is a reasonable default to make it easier for people to get started.

I'll close this then.