[HOTFIX] Fix broken Mima tests on the master branch

By merging #2268, which bumped the Spark version to 1.2.0-SNAPSHOT, I inadvertently broke the Mima binary compatibility tests.  The issue is that we were comparing 1.2.0-SNAPSHOT against Spark 1.0.0 without using any Mima excludes.  The right long-term fix for this is probably to publish nightly snapshots on Maven central and change the master branch to test binary compatibility against the current release candidate branch's snapshots until that release is finalized.

As a short-term fix until 1.1.0 is published on Maven central, I've configured the build to test the master branch for binary compatibility against the 1.1.0-RC4 jars.  I'll loop back and remove the Apache staging repo as soon as 1.1.0 final is available.