upgrade skill-tree to 1.1.0, which has some new features

Author: nikomatsakis

.travis.yml

@@ -15,7 +15,7 @@ rust:
 before_script:
   - (test -x $HOME/.cargo/bin/cargo-install-update || cargo install cargo-update)
   - (test -x $HOME/.cargo/bin/mdbook || cargo install --vers "^0.3" mdbook)
-  - (test -x $HOME/.cargo/bin/skill-tree || cargo install --vers "^1.0" skill-tree)
+  - (test -x $HOME/.cargo/bin/skill-tree || cargo install --vers "^1.1" skill-tree)
   - cargo install-update -a
 
 script: