Bump Rust minimum version to 1.33.0

This is to support the git2 crate update, whose percent-encoding
dependency throws a "local variables in const fn are unstable" error.

.travis.yml

@@ -1,7 +1,7 @@
 language: rust
 rust:
   - stable
-  - 1.31.1
+  - 1.33.0
 
 branches:
   only:

README.md

@@ -17,7 +17,7 @@ For a full overview, along with documentation and installation instructions, vis
 ![Amp screenshot](screenshot.png?raw=true "Amp")
 
 # Installation
-If you have Rust >= 1.31.1 installed:
+If you have Rust >= 1.33.0 installed:
 ```
 cargo install amp
 ```

documentation/pages/installation.md

@@ -45,7 +45,7 @@ brew tap jmacdonald/amp && brew install amp
 !!! info "Supported Release Channels"
     Amp's automated test suite is run using Rust's **stable** release channel;
     beta and nightly release channels are not officially supported. The oldest
-    version of Rust currently supported is **1.31.1**.
+    version of Rust currently supported is **1.33.0**.
 
 1. Install Rust, either through your system's package manager or using [Rust's `rustup` toolchain management utility](https://www.rust-lang.org/en-US/install.html).
 2. Install both the regular and build dependencies listed above.