Update MSRV to 1.31.0 as discussed in rust-lang-deprecated#319

Author: djc

.travis.yml

@@ -1,12 +1,12 @@
 language: rust
 rust:
-    - 1.18.0
+    - 1.31.0
     - stable
     - beta
     - nightly
 cache: cargo
 script:
-  - if [ "$TRAVIS_RUST_VERSION" == "1.18.0" ]; then cp Cargo.lock.ci Cargo.lock; fi
+  - if [ "$TRAVIS_RUST_VERSION" == "1.31.0" ]; then cp Cargo.lock.ci Cargo.lock; fi
   - cargo test
   - cargo test --features backtrace
   - cargo check --no-default-features

README.md

@@ -100,8 +100,8 @@ pub fn read_toolchains(path: PathBuf) -> Result<Toolchains, Error>
 ## Requirements
 
 Both failure and failure_derive are intended to compile on all stable versions
-of Rust newer than 1.18.0, as well as the latest beta and the latest nightly.
-If either crate fails to compile on any version newer than 1.18.0, please open
+of Rust newer than 1.31.0, as well as the latest beta and the latest nightly.
+If either crate fails to compile on any version newer than 1.31.0, please open
 an issue.
 
 failure is **no_std** compatible, though some aspects of it (primarily the