semver syntax: support hyphen ranges

[StefanKarpinski]

In #751 it came up that it would be nice if the semver version spec parsing supported 0.7-1 hyphen range support (as the call it in npm). Quoth @KristofferC:

Adding things to

Pkg.jl/src/versions.jl

Lines 340 to 345 in dfb341a

	const version = "v?([0-9]+?)(?:\\.([0-9]+?))?(?:\\.([0-9]+?))?"
	const ver_regs =
	[
	Regex("^([~^]?)?$version\$") => semver_interval, # 0.5 ^0.4 ~0.3.2
	Regex("^((?:≥)|(?:>=)|(?:=)|(?:<)|(?:=))v?$version\$") => inequality_interval,# < 0.2 >= 0.5,2
	]

should be fairly simple.