deps: bump minimum Rust to 1.23.0 from 1.20.0

1.23.0 is the first Rust release of 2018 and is around half a year old,
which seems old enough to move to. This also lets us bring in encoding_rs
0.8, which includes performance optimizations.

.travis.yml

@@ -59,13 +59,13 @@ matrix:
     # Minimum Rust supported channel. We enable these to make sure ripgrep
     # continues to work on the advertised minimum Rust version.
     - os: linux
-      rust: 1.20.0
+      rust: 1.23.0
       env: TARGET=x86_64-unknown-linux-gnu
     - os: linux
-      rust: 1.20.0
+      rust: 1.23.0
       env: TARGET=x86_64-unknown-linux-musl
     - os: linux
-      rust: 1.20.0
+      rust: 1.23.0
       env: TARGET=arm-unknown-linux-gnueabihf GCC_VERSION=4.8
       addons:
         apt:

CHANGELOG.md

@@ -6,6 +6,9 @@ Releases provided on Github for `x86` and `x86_64` will now work on all target
 CPUs, and will also automatically take advantage of features found on modern
 CPUs (such as AVX2) for additional optimizations.
 
+This release increases the **minimum supported Rust version** from 1.20.0 to
+1.23.0.
+
 **BREAKING CHANGES**:
 
 * When `--count` and `--only-matching` are provided simultaneously, the

README.md

@@ -296,7 +296,7 @@ If you're a **NetBSD** user, then you can install ripgrep from [pkgsrc](http://p
 ```
 
 If you're a **Rust programmer**, ripgrep can be installed with `cargo`.
-* Note that the minimum supported version of Rust for ripgrep is **1.20**,
+* Note that the minimum supported version of Rust for ripgrep is **1.23.0**,
   although ripgrep may work with older versions.
 * Note that the binary may be bigger than expected because it contains debug
   symbols. This is intentional. To remove debug symbols and therefore reduce