fix incorrect pre versions incrementing

I found one strange behaviour of `new_number` method.

Example:

You have a gem with the version "1.0.1.pre19" and you would like to release another pre version, and I'm expecting that it would be "1.0.1.pre20". However, VersionFile does return me "1.0.1.pre110".

```ruby
module MyEngine
  VERSION = "1.0.1.pre19"
end
```

Irb output:
```
1.9.3-p484 :001 > require 'gem_release'
 => true
1.9.3-p484 :002 > GemRelease::VersionFile.new(target: :pre).new_number
 => "1.0.1.pre110"
```

This happens because of incorrect version parsing.

I added a fix and a test case to reproduce it and ensure that it's being fixed.

Would be happy to hear any feedback.