Added SemVer compatibility operator "^"

Dub now supports version specifications of the form `^x.y.z`.
This corresponds to a "semver compatible version", ie. any version up from
`x.y.z` with the same major number. If the major number is 0, only the same
version matches. This corresponds to the versions listed on https://semver.org/ as
compatible with the public API of the version given.
`^x.y` is equivalent to `^x.y.0`.