message: Always force install mdbook.
diff: set -ex
  -do_install() {
  - app=$1
  -
  - # Attempt to install app
  - ret=0
  - cargo install $app || ret=$?
  -
  - # If it failed (most likely already installed) then check versions and update if necessary.
  - if [ 101 = $ret ]; then
  - new_version=`cargo install $app 2>&1 | grep "Installing $app" | grep -o "[0-9.]*" || true`
  - installed_version=`cargo install --list | grep "$app v" | grep -o "[0-9.]*" || true`
  - if [[ "$new_version" != "$installed_version" ]]; then
  - cargo install $app --force
  - fi
  - fi
  -}
  -
  cargo install clippy --force
  if [ "$TRAVIS_OS_NAME" = "linux"]; then
  - do_install mdbook
  + cargo install mdbook --force
  fi
  if [ "$TRAVIS_OS_NAME" = "osx" ] && [ "$GNUPLOT" = "yes" ]; then
  