Drop Rust 1.31 compatibility

Repeated build failures makes it infeasible to keep lipsum compatible
with any particular version of Rust. We will therefore track the
latest stable version of Rust from now on.

.travis.yml

@@ -1,7 +1,6 @@
 language: rust
 
 rust:
-  - 1.31.0 # Rust 2018
   - stable
   - nightly
 

README.md

@@ -79,8 +79,15 @@ This is a changelog with the most important changes in each release.
 
 ### Unreleased
 
-We now use [Rust 2018][rust-2018], which means we require Rust version
-1.31.0 or later.
+We now require the [Rust 2018 edition][rust-2018]. Over the years,
+we’ve repeatedly seen build failures in our CI, even when nothing
+changed in `lipsum`. The failures happened because we tested against a
+fixed version of Rust, but our dependencies kept releasing new patch
+it has versions that would push up the minimum required Rust version.
+
+The build failures makes it infeasible to keep `lipsum` compatible
+with any particular version of Rust. We will therefore track the
+latest stable version of Rust from now on.
 
 ### Version 0.6.0 — December 9th, 2018