Update MSRV to 1.31

Upstream cfg-if now requires Rust 1.31 to build (despite not having done
a major version bump). Updating our MSRV gets CI building correctly
again.

Signed-off-by: Nick Stevens <[email protected]>

Author: nastevens

.travis.yml

@@ -1,7 +1,7 @@
 language: rust
 sudo: false
 rust:
-  - 1.26.0
+  - 1.31.1
   - stable
   - beta
   - nightly

CHANGELOG.md

@@ -5,6 +5,7 @@
 ### Changed
 
 - Migrated to 'tokio' crate. 
+- Minimmum supported Rust version updated to 1.31
 
 ## [0.5.3] - 2018-04-19