re-enabled some replica set tests because we fixed Tokutek/mongo#93

Revert "detect replica sets in ruby tests in a different way"

This reverts commit 2e90694d93d5210529ad126e054f3526d90fdd39.

Revert "disable some tests that insert lots of large documents, when running on a replset"

This reverts commit d7999f0f5bc92645537b64a3eae72676ddd143b3.