Add range and/or sequence syntax for dependency versions

With the near constant state of change in Swift, Xcode and prerelease libraries Carthage may need to support a range and/or sequence syntax for specifying compatible versions. Given that commits have a total order this should be feasible, but I haven't looked closely at the version resolving code to see how complicated this would be.

```
# baseline commit through some branch head
github "ReactiveCocoa/ReactiveCocoa" "v4.0-alpha" => "swift2"
# or a series of commits
github "ReactiveCocoa/ReactiveCocoa" "asdf1234", "zxcv5678", "qwer890a"
# maybe some other syntax, haven't thought too much about it yet
```

Of course this wouldn't magically fix all down-stream breaks when dependencies are poorly specified. However, it would at least give us a tool to better scope and constrain our dependencies without first having to go fix the lower level frameworks.