Export sbt environment variables in 'setup' stage

In #154, export of SBT_OPTS and JVM_OPTS were wrongly added to 'export'
stage, which is *useless* since this stage is executed before 'git clone' of project repository.

I propose to do these exports in 'setup' stage, similarly to `jdk_switcher` call in [jdk.rb](https://github.com/travis-ci/travis-build/blob/master/lib/travis/build/script/jdk.rb#L12)