Fix the build.

After an upstream fix in docopt, cargo now builds on rust HEAD. Also,
move to the semver crate rather than the deprecated built-in one.
auto merge of #306 : steveklabnik/cargo/fix_build, r=alexcrichton

After an upstream fix in docopt, cargo now builds on rust HEAD. Also,
move to the semver crate rather than the deprecated built-in one.