diff --git a/.travis.yml b/.travis.yml
index 5978c61d..36c3719f 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -1,6 +1,8 @@
 language: rust
 
-cache: pip
+cache:
+- pip
+- cargo
 
 install:
   - source ~/.cargo/env || true
diff --git a/ci/install.sh b/ci/install.sh
index dd96daf1..fac6488b 100644
--- a/ci/install.sh
+++ b/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