Leave the generated bindings out of the source repository and instead
have Cargo automatically build them if they are missing. This allows the
version of MagickWand to be incorporated into the bindings
automatically. For instance, in Homebrew the version is 6.Q16, while on
FreeBSD 10.2 it is 6.9, and only be generating the bindings for each
system can we ensure a smooth compilation process.
