Use rust stable in Travis

Currently we require rust 1.31 (beta?) for use with asan, because of the fixes in legacy/trac#27273 (moved).

1. On Travis, we should add a test that uses rust stable without sanitizers.

2. 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.