Cache the cargo-tree installation

Author: dtolnay

.travis.yml

@@ -1,10 +1,14 @@
 sudo: false
 language: rust
+cache: cargo
 
 rust: stable
 
 before_install:
-  - cargo install cargo-tree --debug
+  - |
+      if ! hash cargo-tree 2>/dev/null; then
+          cargo install cargo-tree --debug
+      fi
   - export PATH=$HOME/.cargo/bin:$PATH
 
 script: