Use latest patch release of mdBook and linkcheck

Author: camelid

.travis.yml

@@ -11,8 +11,8 @@ before_install:
 - MAX_LINE_LENGTH=100 bash ci/check_line_lengths.sh src/**/*.md
 install:
 - source ~/.cargo/env || true
-- cargo install mdbook --version 0.4.3
-- cargo install mdbook-linkcheck --version 0.7.0
+- cargo install mdbook --version '^0.4.3'
+- cargo install mdbook-linkcheck --version '^0.7.0'
 script:
 - git checkout -b ci
 - git rebase origin/master