Updated the main `rake test:integration` task given changes in 8d8dd62
[EXT] Improved the testing cluster (Elasticsearch::Extensions::Test::Cluster)

* Gather PIDs during startup and exit(1) unless all processes stayed running
* Do not swallow STDERR when starting the server
* Print the response when the `DEBUG` environment variable is set
[API] Launch only 1 ES node when starting cluster for integration tests automatically
[API] Fixed, that exceptions raised during checking `build_hash` are catched

Also added `TEST_NO_SPEC_CHECKOUT` environment variable to prevent
the automatic checking out of the `build_hash` commit.
[API] Added, that a commit to be checked out during integration tests can be specified with the `TEST_BUILD_HASH` environment variable