Bump minimum rust version

.travis.yml

@@ -5,7 +5,7 @@ sudo: required
 
 matrix:
   include:
-    - rust: 1.23.0
+    - rust: 1.27.2
     - rust: stable
     - rust: beta
     - rust: nightly

README.md

@@ -24,10 +24,10 @@ fn main() {
 
 See the [docs](https://docs.rs/base64) for all the details.
 
-Purpose
+Rust version compatibility
 ---
 
-I have a fondness for small dependency footprints, ecosystems where you can pick and choose what functionality you need, and no more. Unix philosophy sort of thing I guess, many tiny utilities interoperating across a common interface. One time making a Twitter bot, I ran into the need to correctly pluralize arbitrary words. I found on npm a module that did nothing but pluralize words. Nothing else, just a couple of functions. I'd like for this to be that "just a couple of functions."
+The minimum required Rust version is 1.27.2.
 
 Developing
 ---