Raise minimum supported Rust version to 1.25.0.

The `backtrace` crate has this as a minimum required version since
0.3.13.

Author: azriel91

.travis.yml

@@ -1,6 +1,6 @@
 language: rust
 rust:
-    - 1.18.0
+    - 1.25.0
     - stable
     - beta
     - nightly

Cargo.toml

@@ -15,7 +15,7 @@ path = "./failure_derive"
 
 [dependencies.backtrace]
 optional = true
-version = "=0.3.9"
+version = "0.3.3"
 
 [workspace]
 members = [".", "failure_derive"]

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 1.25.0, as well as the latest beta and the latest nightly.
+If either crate fails to compile on any version newer than 1.25.0, please open
 an issue.
 
 failure is **no_std** compatible, though some aspects of it (primarily the