[PodXCConfig] Use recursive dependent targets

[InstallationOptions] Add #share_schemes_for_development_pods

This allows for opting out of sharing schemes for development pods.
By default, the current behavior is kept -- all dev pods have their
schemes shared. By setting the option to `nil` or `false`, no schemes
are shared. Otherwise, an array can be passed, and only dev pod names that
match elements of that array will have their schemes shared.

Improve messaging when dependencies conflict

[Travis] Update RubyGems

[Validator] Validate that resource bundles contain resources

[CHANGELOG] Fix broken links to issues

[Resolver] Handle when an abstract target has no platform