…atype OSS releases)

Use https instead of http for JGit resolver, otherwise sbt gives security warnings.

See also:

https://www.scala-sbt.org/1.x/docs/Resolvers.html#Predefined+resolvers

This project now compiles with Scala 2.13 by default, while still
cross-compiling to Scala 2.12 & 2.11.

Although Scala 2.13.1 has been released, this change sticks with 2.13.0,
as `scoverage` has not yet released an update compatible with 2.13.1:

scoverage/sbt-scoverage#295
scoverage/scalac-scoverage-plugin#283
scoverage/sbt-scoverage#299


Migration issues addressed with the update to Scala 2.13:

### Underscore (`_`) is no longer a valid identifer for `val`s

Using underscore like this is no longer allowed...

```
implicit val _ = ...
```

...we have to give the `val` a valid name (even if it's just `v`):


```
implicit val v = ...
```

See also:

* scala/bug#10384
* scala/bug#7691
* scala/scala#6218

### `Map.mapValues()` now returns a `MapView` rather than a `Map`

We usually want a `Map`, so we can just add in a `.toMap` to the end of
the expression - it's harmless in Scala 2.12, and allows the code to
compile in Scala 2.13. `Map.mapValues()` itself is deprecated in Scala
2.13, but using the new recommended alternative doesn't work in Scala 2.12.

https://docs.scala-lang.org/overviews/core/collections-migration-213.html#what-are-the-breaking-changes

See also:

* scala/bug#10919
* scala/scala#7014

### `.to` can no longer infer what resulting collection type you want

This is all part of `CanBuildFrom` going away in Scala 2.13:

https://www.scala-lang.org/blog/2018/06/13/scala-213-collections.html#life-without-canbuildfrom