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 <nick@bitcurry.com>

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
 

README.md

@@ -84,6 +84,11 @@ The following features are planned for the library:
 - [x] Support for asynchronous polling using `mio` or `tokio` (requires
       enabling the `mio-evented` or `use_tokio` crate features, respectively)
 
+## Minimum Supported Rust Version (MSRV)
+
+This crate is guaranteed to compile on stable Rust 1.31 and up.  It *might*
+compile with older versions but that may change in any new patch release.
+
 ## Cross Compiling
 
 Most likely, the machine you are running on is not your development