Test on rust 1.0.0, nightly, and beta

.travis.yml

@@ -1,4 +1,8 @@
 language: rust
+rust:
+  - 1.0.0
+  - beta
+  - nightly
 sudo: false
 script:
   - cargo build --verbose
@@ -8,6 +12,7 @@ script:
 after_success: |
   [ $TRAVIS_BRANCH = master ] &&
   [ $TRAVIS_PULL_REQUEST = false ] &&
+  [ $TRAVIS_RUST_VERSION = nightly ] &&
   echo '<meta http-equiv=refresh content=0;url=gcc/index.html>' > target/doc/index.html &&
   pip install ghp-import --user $USER &&
   $HOME/.local/bin/ghp-import -n target/doc &&