Correctly handle `beta` and `pre` gems

[joonty]

If a gem version is in beta or pre (the final part of the version can be any string, I think), pessimize doesn't currently work correctly. For example, the debugger2 gem has version "1.0.0.beta2", but using the constraint "~> 1.0" doesn't seem to include beta gems.

There are two ways round this:

1. Don't add constraints to beta/pre gems, i.e. gem "debugger2"
2. Specify the exact version for beta/pre gems, i.e. gem "debugger2", "1.0.0.beta2"

I'm leaning towards 1, as it's not ideal to lock down a gem version to always be in beta.

[LimeBlast]

Maybe, if the script comes across such a gem, you could add some text to the output explaining this issue, and let the user choose what's best for them.

Something in beta is way more likely to have breaking changes, but it always depends on the context, and an informed user can make a decision based their own context.

I think it might also be worth stressing the pros and cons of each approach, i.e. the first one will allow for breaking changes, but you'll always be up to date, while the second one will always work as it'll never change - or something like that.

[joonty]

Nice idea, thanks @LimeBlast.