Make warning flags less aggressive: "Warn and treat as error" -> "Warn"

The following aggressive flags:

* `GCC_WARN_ABOUT_RETURN_TYPE: YES_ERROR`
* `CLANG_WARN_DIRECT_OBJC_ISA_USAGE: YES_ERROR`
* `CLANG_WARN_OBJC_ROOT_CLASS: YES_ERROR`

Break some Pods.

While in theory the concerned libraries could be updated, this happens with non-current, non-longer maintained versions that will see no further updates (for instance PhoneGap/Cordova 2.x).

The only current solution is to manually clear the flags but it has to be done every time a pod command is used.

This commit replaces those flags for regular warnings (non-treated as errors), which fulfills the original goal (highlight library potential issues) while not braking them.

Related to https://github.com/CocoaPods/CocoaPods/issues/1629.