Temporarily depend on Jenkins 1.560 for Java 8 unit testing

A blocking bug in the LTS version prevents tests from running on Java
8.  That bug is fixed in the most recent release candidate
(1.554.1-rc).

Once the release candidate has released, this can change to depend on
1.554.  The released plugin can't be forced to depend on 1.554.1
because that will prevent users of earlier, stable Jenkins versions
from upgrading the plugin.  Running tests with Java 8 on the master
branch will probably need to wait a year until 1.554 becomes the
widely supported oldest version.
Add README comments about the purpose of this branch
Use latest Jenkins LTS for JDK 8 support