Enable simultaneous test runs by interpolating TEST_ENV_NUMBER into simplecov's dir name

Otherwise, the test runners overwrite each other's coverage data, producing false negatives.

TEST_ENV_NUMBER is the variable used in parallel_tests for database names, etc.
Get the tests running on Rubinius 2

I'm having trouble getting simplecov to work with Rubinius's coverage stdlib,
so I'm skipping that for now.