Require nightly Rust due to use of before_exec

.travis.yml

@@ -8,10 +8,10 @@ env:
 matrix:
   include:
     - os: linux
-      rust: stable
+      rust: nightly
       env: TARGET=x86_64-unknown-linux-gnu
     - os: osx
-      rust: stable
+      rust: nightly
       env: TARGET=x86_64-apple-darwin
 
 script:

appveyor.yml

@@ -3,7 +3,7 @@ environment:
     PROJECT_NAME: watchexec
   matrix:
     - TARGET: x86_64-pc-windows-gnu
-      CHANNEL: stable
+      CHANNEL: nightly
 
 # Install Rust and Cargo
 # (Based on from https://github.com/rust-lang/libc/blob/master/appveyor.yml)