Several build updates, plus a Scala 2.11.0-RC1 build.

* Add a 2.11.0-RC1 build; its only difference from other versions is
  that unused imports are warned about.

* Only one 2.10.x cross-build is needed; remove redundant one.

* Build standard against 2.10.3.

* Update scalacheck as far as possible for each supported scala version.

* Infix ?= is wonky for scala 2.9.2+scalacheck 1.10.1; use function
  version instead.  It becomes easy to use ?= again once 2.9.2 support
  is dropped.

* Update sbt to 0.13.1.

* Update bintray-sbt to 0.1.1.