[asm-cherry-pick] [move to start] Use JDK8 to build

[retronym]

Our Travis config no longer seems to work to install and switch to JDK6.

Rather than messing around with this, I propose we just build with Java 8. We are already using -target 1.6.

The risk is that we start using an API that only exists in Java 8, which cause a breakage under Scala 2.11.12 on Java 6/7.

We could mitigate that risk by include a manual step in our release process to compile locally with JDK6.

[retronym]

@lrytz I'm going to optimistically merge this to get the first build of 6.2 published. Feel free to debate the best course of action and we can amend as needed.

[lrytz]

OK for now, but we should probably fix this. We're using openjdk 6 for other modules too, when building for 2.11.

It used to work when releasing 6.0. At the time, travis was already using trusty. However, looking at the build log of the 6.0-scala-1 build (https://travis-ci.org/scala/scala-asm/builds/303559832), it seems the build runs jdk_switcher use openjdk6 (with a message "directory does not exist: /usr/lib/jvm/java-1.6.0-openjdk-amd64") before actually installing the apt package. Even so, the java version ended up being 6.

In the build of 6.2-scala-1 (https://travis-ci.org/scala/scala-asm/builds/389051634), the order of events is the same, but the result is that java 8 is on.

I'll file a ticket / request. It seems an alternative would be to go back to dist: precise, but that should not be necessary.

[lrytz]

Filed travis-ci/travis-ci#9713