Ensure normalized versions uses in less than and greater than

The less than and greater than operators of Specifier were not
correctly ensuring that they normalized the version before doing
any comparisons. This caused things like 2.0.5 to not be included
as part of 2.0dev while it was included in 2.0.dev.
Merge pull request #18 from dstufft/ensure-normalized

Ensure normalized versions uses in less than and greater than