Revert "Lock to older rdiscount."

This reverts commit 0794c08c8dc43ce2b0ad31bff17053f9c3285b11.

The problem is actually that rdiscount 2.0.7 won't compile with Ruby
1.9.2. Switching up to 1.9.3 in CI is reasonable and reflects real-
world use.
Support for building with AS 3.2.