Update Travis from trusty to xenial, and to Rust 1.33.

.travis.yml

@@ -7,7 +7,7 @@ language: rust
 rust:
     # The oldest version we currently support. See
     # CONTRIBUTING.md#rustc-version-support for details.
-    - 1.32.0
+    - 1.33.0
     - beta
     - nightly
 matrix:
@@ -19,7 +19,7 @@ matrix:
     # Similarly, we don't need to hold up people using stable while we wait
     # for the results which may fail.
     fast_finish: true
-dist: trusty
+dist: xenial
 sudo: false
 addons:
     apt:

cranelift/README.md

@@ -10,7 +10,7 @@ into executable machine code.
 [![Travis Status](https://travis-ci.org/CraneStation/cranelift.svg?branch=master)](https://travis-ci.org/CraneStation/cranelift)
 [![Appveyor Status](https://ci.appveyor.com/api/projects/status/oub7wrrb59utuv8x?svg=true)](https://ci.appveyor.com/project/CraneStation/cranelift)
 [![Gitter chat](https://badges.gitter.im/CraneStation/CraneStation.svg)](https://gitter.im/CraneStation/Lobby)
-![Minimum rustc 1.31](https://img.shields.io/badge/rustc-1.31+-green.svg)
+![Minimum rustc 1.33](https://img.shields.io/badge/rustc-1.33+-green.svg)
 
 For more information, see [the
 documentation](https://cranelift.readthedocs.io/en/latest/?badge=latest).