Fix #1206: Update to sbt 0.13.6

Also fix the sbt version for the sbt-plugin-test build.
Merge pull request #1213 from gzm0/sbt-0.13.6

Fix #1206: Update to sbt 0.13.6
Fix #1208: toString for Semantics and CheckedBehaviors

Looks like this:

    Semantics(
      CheckedBehaviors(
        asInstanceOfs = Unchecked
      ),
      strictFloats = false
    )
Fix #1212: Run CI against Java 1.{6-8}
Merge pull request #1220 from gzm0/sem-tostring

Fix #1208: toString for Semantics and CheckedBehaviors
Buglist partests arrayclone-{old,new} and issue192.

They're broken on master.
Merge pull request #1221 from gzm0/cross-jdk

Fix #1212: Run CI against Java 1.{6-8}
Merge pull request #1223 from sjrd/buglist-broken-partests

Buglist partests arrayclone-{old,new} and issue192.
Fix #1222: The optimizer breaks Array.clone().
Blacklist run/issue192.scala because it relies on strict floats.
Merge pull request #1225 from sjrd/fix-reachability-array-clone

Fix broken partests
Fix #1228: Increase partest build timeout to 10h
Merge pull request #1229 from gzm0/partest-timeout

Fix #1228: Increase partest build timeout to 10h
Fix #1224: Don't use deprecated reporting methods
Merge pull request #1231 from gzm0/compiler-deprecation

Fix #1224: Don't use deprecated reporting methods
Blacklist neg/t6289 (uses .java source files)
Run partests on JDK6 with Scala 2.11.1 (see #1227)
Merge pull request #1232 from gzm0/blacklist-neg_t6289

Blacklist neg/t6289 (uses .java source files)
Merge pull request #1230 from gzm0/move-jdk6-partest-version

Run partests on JDK6 with Scala 2.11.1 (see #1227)
Don't run partests on JDK6.

Scalac in 2.11.x is unable to compile partest on Oracle JDK6. We
therefore do not run any partests. This closes #1227.