Skip Splinter tests on Travis and ci-new (Jenkins)

Now when the environment variable SKIP_SPLINTER_TESTS=1 it will not instantiate a SplinterClient and those tests are skipped.

That that skipping tests using tags does not prevent their context from being created, meaning we still instantiate ie a SplinterClient for skipped tests(!) Our python code *also* has to change to get round this... yuk.