Improved sanitizing of configuration names which have a number prefix.

This pull request adds an underscore before the preprocessor definition if the configuration name has a number prefix.

The previous bugfix  https://github.com/CocoaPods/CocoaPods/commit/e11291427d08b05bc13b801b6f3ded6fd36afd0a doesn't covers configuration names with a number prefix.
Can you replace `gsub` with `sub` ? The former is the greedy variant, but since this only ever replaces one occurrence, it doesn’t need that behaviour.