Remove @RunWith annotations from test cases.

These are are at odds with the junit-interface SBT test framework
under SBT 0.13.+, and appear to be superfluous.
Update to SBT 0.13.0-M5

Which correctly substitutes scala-reflect.jar when using scalaHome
to test out the plugin with a local version of Scala.

Also updates to a released version of the junit-interface SBT TestFramework.
Better support for testing with a local instance of Scala.

SBT's scalaHome handling doesn't pick up the local continuations
plugin.
Use @uncheckedBounds to avoid introducing refchecks errors

... in code that would otherwise have smuggled through these
slack LUBs in the types of trees but never in a TypeTree.

More details in SI-7694.

Fixes #29
Merge pull request #30 from retronym/topic/unchecked-bounds

Use @uncheckedBounds to avoid introducing refchecks errors …