Fix dependencies for ruby 1.9.3

This is a candidate fix for https://github.com/ruby-debug/ruby-debug-ide/issues/56.

If ruby 1.9.3 is detected, it will force the usage to 0.11.30.pre15 or later. I admit it isn't a perfect solution for those who would like only "stable" gems but I think that, pragmatically, it's an okay fix considering that:

- those gems are usually installed in the context of development
- the ruby-debug-base19x gems have remained in the prerelease state for a long time

Also, on a side note, I was tempted to move the dependency code in the begin block, so all that if block doesn't get executed at all if the current ruby gem dependency requirements are met, but I stopped there, aiming for a minimal fix to facilitate a quick review...