Update .travis.yml

Bumped mdbook version due to compilation failure on latest rust toolchain

.travis.yml

@@ -9,7 +9,7 @@ rust:
 
 before_script:
   - (test -x $HOME/.cargo/bin/cargo-install-update || cargo install cargo-update)
-  - (test -x $HOME/.cargo/bin/mdbook || cargo install --vers "^0.3" mdbook)
+  - (test -x $HOME/.cargo/bin/mdbook || cargo install --vers "^0.4" mdbook)
   - cargo install-update -a
 
 script:
@@ -22,4 +22,4 @@ deploy:
   local-dir: ./book
   keep-history: false
   on:
-    branch: master
+    branch: master