Change the last supported Rust version to 1.31.0

.appveyor.yml

@@ -26,7 +26,7 @@ environment:
 
   matrix:
   # minimum version
-  - CHANNEL: 1.27.0
+  - CHANNEL: 1.31.0
     ARCH: i686
     ABI: msvc
   # "msvc" ABI

.travis.yml

@@ -17,7 +17,7 @@ matrix:
     - rust: nightly
   fast_finish: true
   include:
-    - rust: 1.27.0
+    - rust: 1.31.0
     - rust: stable
       os: linux
       env: TEST_INSTALL=true

README.md

@@ -41,7 +41,7 @@ Requirements
 ### Rust Version ###
 
 uutils follows Rust's release channels and is tested against stable, beta and nightly.
-The current oldest supported version of the Rust compiler is `1.27.0`.
+The current oldest supported version of the Rust compiler is `1.31.0`.
 
 On both Windows and Redox, only the nightly version is tested currently.