NpmSpec Comparison Between Prerelease Versions is Wrong

Version is 2.8.5.

```
>>> from semantic_version import NpmSpec, Version
>>> NpmSpec("<0.1.1-alpha.1").match(Version("0.1.1-beta.1"))
True
>>> NpmSpec("<0.1.1-alpha.1").match(Version("0.1.1-rc.1"))
True
>>> NpmSpec("<0.1.1-beta.7").match(Version("0.1.1-beta.8"))
True
```

They should be false.
