Give `mutants` a version 0.1.0 (eventually)

[kpreid]

If the mutants macro package is going to be updated more often than never, it would be good to give the next such release a version number which is at least 0.1.0. This is because Cargo considers 0.0.3 and 0.0.4 to be incompatible (compatibility is determined by the first nonzero component of the version number), and so cargo update will not update a dependency on 0.0.3 to 0.0.4, whereas 0.1.0 has room for compatible releases 0.1.1 and onward.