Make 'fastlane' a development_dependency in badge.gemspec

[liamnichols]

Resolved #97

Thanks for all the prior work that has gone into this library, we've used it for a long time now and it's ace 🙏🙏🙏

As per my description in #97, this change marks the fastlane gem as a development dependency rather than a runtime dependency so this should break the cycle that people may be running into.

[liamnichols]

Note that it turns out the issue that I was originally referring to was in fact caused by fastlane/fastlane/issues/16130, but I was led to believe that it was related to fastlane-plugin-badge since it was depending on fastlane..

I think (unless I've misunderstood something) that its still good practice to keep this just as a development dependency but please do correct me if I'm wrong 🙏

[HazAT]

I won't merge this, I know it's ideal but this plugin was designed to be used with fastlane.