Update rust min version for travis

Author: johnteee

.travis.yml

@@ -46,13 +46,13 @@ matrix:
     # Minimum Rust supported channel. We enable these to make sure ripgrep
     # continues to work on the advertised minimum Rust version.
     - os: linux
-      rust: 1.23.0
+      rust: 1.27.0
       env: TARGET=x86_64-unknown-linux-gnu
     - os: linux
-      rust: 1.23.0
+      rust: 1.27.0
       env: TARGET=x86_64-unknown-linux-musl
     - os: linux
-      rust: 1.23.0
+      rust: 1.27.0
       env: TARGET=arm-unknown-linux-gnueabihf GCC_VERSION=4.8
       addons:
         apt: