travis: Stop Pinning nightly

Signed-off-by: Joe Richey <joerichey@google.com>

.travis.yml

@@ -91,7 +91,7 @@ matrix:
         - RUSTFLAGS='-C debuginfo=0' cargo test --target asmjs-unknown-emscripten
 
     - name: "Linux, nightly, docs"
-      rust: nightly-2020-09-08
+      rust: nightly
       os: linux
       install:
         - cargo --list | egrep "^\s*deadlinks$" -q || cargo install cargo-deadlinks
@@ -109,7 +109,7 @@ matrix:
         - cargo test
 
     - name: "OSX, nightly, docs"
-      rust: nightly-2020-09-08
+      rust: nightly
       os: osx
       install:
         - cargo --list | egrep "^\s*deadlinks$" -q || cargo install cargo-deadlinks
@@ -127,7 +127,7 @@ matrix:
         - cargo test
 
     - name: "cross-platform build only"
-      rust: nightly-2020-09-08
+      rust: nightly
       install:
         - rustup target add x86_64-sun-solaris
         - rustup target add x86_64-unknown-freebsd