tests: Make the server version check more forgiving

Instead of requiring the server to have the exact version number of
2.0.0, allow anything that starts with 2.0. This makes the test suite
work with Riak 2.0.1.

Signed-off-by: Gergely Nagy <algernon@madhouse-project.org>
Merge pull request #2 from algernon/h/tests/version-check-too-strict

tests: Make the server version check more forgiving