Reduce strain on CI

This moves 8 tasks from the PR CI configuration to the nightly and
moves most partest tasks from the nightly to a new weekly build.

N.B.: We should not be doing this, but given the load on the CI we
probably have no choice. The full tests should be introduced ASAP.
Refactor JSEnv to support async running
Remove JSEnv constructor-blow up. Use default args
Merge pull request #1238 from gzm0/reduce-ci-strain

Reduce strain on CI
Add ComJSEnv and make RhinoJSEnv a ComJSEnv
Make NodeJSEnv a ComJSEnv
Clean up standalone sbt test project
Restrict files from plugin we include in partest build
Make preLinkJSEnv and postLinkJSEnv a task.

They might depend on a task such as update. On the other hand, they
are only used to run things, which arguably must be a task. Therefore,
it is safe to make them tasks.
Make PhantomJSEnv a ComJSEnv
Remove failing bootstrapping test.

The tools do not compile on JDK6 with Scala 2.11.x. (see #1235)
Merge pull request #1237 from gzm0/com-js-env

Add ComJSEnv
Merge pull request #1243 from gzm0/remove-jdk6-2.11-tools

Remove failing bootstrapping test.