improvements to travis setup -- cache, use command -v

Author: nikomatsakis

.travis.yml

@@ -1,6 +1,8 @@
 language: rust
 
-cache: pip
+cache:
+- pip
+- cargo
 
 install:
   - source ~/.cargo/env || true

ci/install.sh

@@ -1,7 +1,9 @@
 #!/bin/bash
 set -ex
 
-cargo install cargo install mdbook --vers "0.0.28"
+if command -v mdbook >/dev/null 2>&1; then
+    cargo install mdbook --vers "0.0.28"
+fi
 
 if command -v ghp-import >/dev/null 2>&1; then
     pip install ghp-import