assorted version bumps

[SethTisue]

specific motivation: scala/community-build#862 , but while I'm here I threw in other version bumps as well

[SethTisue]

[info] 'compiler-interface' not yet compiled for Scala 2.12.0. Compiling...
error: scala.reflect.internal.MissingRequirementError: object java.lang.Object in compiler mirror not found.
	at scala.reflect.internal.MissingRequirementError$.signal(MissingRequirementError.scala:17)
	at scala.reflect.internal.MissingRequirementError$.notFound(MissingRequirementError.scala:18)

that's because the cross-build includes minor Scala versions such as 2.12.0, which are too old to be JDK 11 friendly

[SethTisue]

I think anyone who's stuck on an old 2.11.x or 2.12.x version should also be content to be stuck on an old scala-continuations version, so I've pushed an additional commit that drops everything older than 2.11.12 and 2.12.6

[SethTisue]

ffs. in the JDK 6 job,

0.06s$ java -version
java version "1.6.0_41"
OpenJDK Runtime Environment (IcedTea6 1.13.13) (6b41-1.13.13-0ubuntu0.12.04.1)
OpenJDK 64-Bit Server VM (build 23.41-b41, mixed mode)
The command "java -version" exited with 0.
7.59s$ admin/build.sh
Downloading sbt launcher for 0.13.18:
  From  http://repo.typesafe.com/typesafe/ivy-releases/org.scala-sbt/sbt-launch/0.13.18/sbt-launch.jar
    To  /home/travis/.sbt/launchers/0.13.18/sbt-launch.jar
*** buffer overflow detected ***: java terminated
======= Backtrace: =========
/lib/x86_64-linux-gnu/libc.so.6(__fortify_fail+0x37)[0x7ffaa2020db7]
/lib/x86_64-linux-gnu/libc.so.6(+0x10bcb0)[0x7ffaa201fcb0]
/usr/lib/jvm/java-6-openjdk-amd64/jre/lib/amd64/libnet.so(Java_java_net_Inet4AddressImpl_getLocalHostName+0x1a0)[0x7ffa94f13df0]
[0x7ffa985a86b0]

[SethTisue]

well that needn't block merging of this, since it's a progression, not a regression: previous runs such as https://travis-ci.org/scala/scala-continuations/jobs/456274852 were asking for JDK 6 and getting JDK 8 instead, so before this PR we had no actual JDK 6 runs either

[SethTisue]

I opened #43 on the JDK 6 thing