change package name regex in `Pkg.clone`

Yesterday @tkelman was having problems running a `Pkg.clone` on a windows box ([here](https://github.com/JuliaLang/JSON.jl/pull/54)). I started poking at it and found this:

```julia
julia> pwd()
"C:\\foo\\Bar.jl"

julia> Pkg.clone(pwd())
Error: can't determine package name from URL: C:\foo\Bar.jl
...
```

Looking at the regex changed here, it looks like there's a rather arbitrary "/" thrown in which prevents it from almost ever working on windows... After removal (and a little testing here: http://regex101.com/), as far as I can tell, old paths / urls still work and new ones without "/" do too.