switch Travis to using Rust stable
Right now our Travis use Rust nightly instead of stable. Now that we are standardizing on a stable Rust (see #24765 (moved)), we should have Travis use stable so we catch accidental uses of nightly features.
Right now our Travis use Rust nightly instead of stable. Now that we are standardizing on a stable Rust (see #24765 (moved)), we should have Travis use stable so we catch accidental uses of nightly features.