add travis.yml

Author: steveklabnik

.travis.yml

@@ -0,0 +1,13 @@
+language: rust
+
+cache: cargo
+
+rust:
+  - nightly
+  - beta
+  - stable
+
+before_script:
+  - (cargo install mdbook --git https://github.com/azerupi/mdBook.git --force || true)
+
+script: /home/travis/.cargo/bin/mdbook test;