Update build machinery and README file with 1.31 Rust version requirement.

Author: brandonedens

.travis.yml

@@ -1,7 +1,7 @@
 language: rust
 sudo: false
 rust:
-  - 1.26.0  # MSRV
+  - 1.31.0  # MSRV
   - stable
   - beta
   - nightly

README.md

@@ -76,8 +76,8 @@ The following features are implemented and planned for the library:
 
 ## Minimum Supported Rust Version (MSRV)
 
-This crate is guaranteed to compile on stable Rust 1.26 and up.  It *might*
-compile with older versions but that may change in any new patch release.
+This crate is guaranteed to compile on stable Rust 1.31 and up (as it requires
+marked as edition 2018).
 
 ## Cross Compiling