Add jdk_switcher Travis hack

[ashawley]

Seems the openjdk6 build on Travis was still running on Java 8. We were missing some special sauce.

Stolen from scala/scala-parser-combinators#156

[ashawley]

Two out of three Java 6 builds succeded, but one did fail with:

:::: ERRORS
	Server access Error: Received fatal alert: protocol_version url=https://repo1.maven.org/maven2/org/scala-sbt/sbt/0.13.17/sbt-0.13.17.pom
	Server access Error: Received fatal alert: protocol_version url=https://repo1.maven.org/maven2/org/scala-sbt/sbt/0.13.17/sbt-0.13.17.jar
	Server access Error: Connection reset url=https://repo.typesafe.com/typesafe/ivy-releases/org.scala-sbt/sbt/0.13.17/ivys/ivy.xml
	Server access Error: Connection reset url=https://repo.scala-sbt.org/scalasbt/ivy-snapshots/org.scala-sbt/sbt/0.13.17/ivys/ivy.xml
:: USE VERBOSE OR DEBUG MESSAGE LEVEL FOR MORE DETAILS
unresolved dependency: org.scala-sbt#sbt;0.13.17: not found
Error during sbt execution: Error retrieving required libraries
  (see /home/travis/.sbt/boot/update.log for complete log)
Error: Could not retrieve sbt 0.13.17

[ashawley]

Ever since Maven central discontinued TLSv1.1 and earlier on June 18th, the build on Travis should have been broken. Further, the tests were running against Java 8. They should have been using Java 6.

We need to take down our green "passing" badge, and put up a "red" failing badge.

It's not often you merge a failing build, but this is just such a case.

Fixing the build failure will continue in #237 or others.