Use rust stable in Travis
Currently we require rust 1.31 (beta?) for use with asan, because of the fixes in #27273 (moved).
On Travis, we should add a test that uses rust stable without sanitizers.
Once Rust 1.31 is stable (December?) we should enable sanitizers with rust stable.
See Teor's https://trac.torproject.org/projects/tor/ticket/27273#comment:14 for a much more comprehensive version of the matrix.