* Backport style changes in tests and Defaults.scala
This backports the scripted tests and Defaults.scala style changes to
use `build.sbt` and `:=`.
* Fix backport
* Deprecate the old operators `<<=`, `<+=`, and `<++=`
The no-longer-documented operators `<<=`, `<+=`, and `<++=` are
deprecated in anticipation of the removal in sbt 1.0. Ref #2711
For `<<=`, the suggested migration would be to use either `:=` or `~=`
operators. The RHS of `<<=` takes an `Initialize[_]` expression, which
can be converted to `:=` style by wrapping the expression in
parenthesis, and calling `.value` at the end.
eugene yokota