Ranges and pre-release bug

[mattfarina]

Consider the case of a range > 0 and the versions 0.0.1-beta and 0.

This range will return false for both. But, isn't 0.0.1-beta greater that 0? If a range like > 0-a is used than a version like 0 doesn't work. It's one of those edge cases when #21/#23 was fixed.

@technosophos this came about when I was trying to update helm for the latest release to deal with the pre-release issue.

Considering two options:

1. Handling this special edge case in the code.
2. Adding a flag to the constraints struct for UsingPrerelease to force it into place without putting a prerelease on the constraint.

@technosophos @sdboyer thoughts?

[technosophos]

Yeah, you're right. It does seem like any non-zero value should satisfy > 0.

[sdboyer]

This sort of thing is why I've imagined the implementation needs to have special values that represent the high (infinity) and low (zero) bounds.

[mattfarina]

PR on #25