Update mdbook

Author: ehuss

.travis.yml

@@ -11,7 +11,7 @@ before_install:
 - MAX_LINE_LENGTH=100 bash ci/check_line_lengths.sh src/**/*.md
 install:
 - source ~/.cargo/env || true
-- cargo install mdbook --version 0.4.0
+- cargo install mdbook --version 0.4.1
 - cargo install mdbook-linkcheck --version 0.7.0
 script:
 - git checkout -b ci