Add Scala 2.13.1 build

[valencik]

This PR adds 2.13.1 to the cross build.

In doing so it does a number of things. It updates some plugins, Scala versions in build.sbt and Travis, updates sbt, and scalatest.

This might change more things than necessary in the build.sbt.

There was some type resolution funniness about Parameter as we have two, one in IR and one in Real, I renamed the IR one to IParameter to make things easier on me. We may not want this change permanently.

Currently this PR ignores the parallel param to the Model.sample as Scala 2.13 broke out the parallel collections into different library. I don't know if dropping parallel is acceptable here.

[codecov-io]

Codecov Report

Merging #459 into 0.3-dev will increase coverage by 0.79%. The diff coverage is 28.57%.

@@             Coverage Diff             @@
##           0.3-dev     #459      +/-   ##
===========================================
+ Coverage    52.18%   52.97%   +0.79%     
===========================================
  Files           73       72       -1     
  Lines         2842     2775      -67     
  Branches       161      169       +8     
===========================================
- Hits          1483     1470      -13     
+ Misses        1359     1305      -54

Impacted Files	Coverage Δ
...n/scala/com/stripe/rainier/compute/BoundsViz.scala	0% <ø> (ø)	:arrow_up:
...scala/com/stripe/rainier/ir/CompiledFunction.scala	92% <ø> (-0.16%)	:arrow_down:
...in/scala/com/stripe/rainier/compute/Compiler.scala	100% <ø> (ø)	:arrow_up:
...la/com/stripe/rainier/ir/ExprMethodGenerator.scala	96.96% <ø> (ø)	:arrow_up:
...pute/src/main/scala/com/stripe/rainier/ir/IR.scala	100% <ø> (ø)	:arrow_up:
...n/scala/com/stripe/rainier/compute/Evaluator.scala	58.33% <0%> (-5.31%)	:arrow_down:
...in/scala/com/stripe/rainier/core/Categorical.scala	33.33% <0%> (-1.45%)	:arrow_down:
...e/src/main/scala/com/stripe/rainier/ir/IRViz.scala	0% <0%> (ø)	:arrow_up:
...src/main/scala/com/stripe/rainier/core/Model.scala	55.55% <100%> (+1.16%)	:arrow_up:
...c/main/scala/com/stripe/rainier/compute/Real.scala	85.21% <100%> (ø)	:arrow_up:
... and 22 more

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[valencik]

@avibryant Now that this is building successfully in CI we just need confirmation on the IParameter change and whether we can drop parallel collection support in Model.sample.

If we can drop parallel collection support I'll clean up that area, and if not we'll have to bring in the parallel collections lib.