Handle `~x.y.z` dependencies

[bvssvni]

See http://www.steveklabnik.com/semver/semver/index.html

~1.2.3 := >=1.2.3 <1.3.0
~1.2   := >=1.2.0 <1.3.0
~1     := >=1.0.0 <2.0.0