Require a current Rust version.
Require a current Rust version.

Rust 1.12 did work at one point, but Travis is testing against the
current stable and beta releases, so that is the only versions we can
claim to support.

Fixes #51.

stoklund/cretonne,stoklund/cretonne,sunfishcode/cretonne,stoklund/cretonne,sunfishcode/cretonne,sunfishcode/cretonne