Unpin nightly version in CI.

Removes temporary workaround for rust-lang/rust#49033.

Author: ehuss

.travis.yml

@@ -28,7 +28,7 @@ matrix:
 
     - env: TARGET=x86_64-unknown-linux-gnu
            ALT=i686-unknown-linux-gnu
-      rust: nightly-2018-03-07
+      rust: nightly
       install:
         - mdbook --help || cargo install mdbook --force
       script:

appveyor.yml

@@ -6,7 +6,7 @@ environment:
 
 install:
   - appveyor-retry appveyor DownloadFile https://win.rustup.rs/ -FileName rustup-init.exe
-  - rustup-init.exe -y --default-host x86_64-pc-windows-msvc --default-toolchain nightly-2018-03-07
+  - rustup-init.exe -y --default-host x86_64-pc-windows-msvc --default-toolchain nightly
   - set PATH=%PATH%;C:\Users\appveyor\.cargo\bin
   - rustup target add %OTHER_TARGET%
   - rustc -V