Fix the build

We can't use the cache dir because cargo install doesn't really have a
way to say "make sure this is installed". If it is, then it errors out,
and you can force it to install, but then we're not using the cache.

.travis.yml

@@ -1,11 +1,10 @@
 sudo: false
 language: rust
-cache: cargo
 rust:
   - nightly
   - stable
 before_script:
-        - cargo install mdbook --git https://github.com/azerupi/mdBook.git
+  - cargo install mdbook
 script:
   - PATH=$PATH:/home/travis/.cargo/bin mdbook test
   - PATH=$PATH:/home/travis/.cargo/bin mdbook build