Updating outdated deps

[tesuji]

The contributor doing this may want to look at each dep changelog
and link it in PR body.

$ cargo outdated
Name                   Project  Compat  Latest   Kind    Platform
----                   -------  ------  ------   ----    --------
cfg-if                 0.1.10   ---     1.0.0    Normal  ---
pulldown-cmark         0.7.2    ---     0.8.0    Normal  ---
semver                 0.10.0   ---     0.11.0   Normal  ---
semver->semver-parser  0.7.0    ---     0.10.0   Normal  ---
...