You should keep the AdSupport framework in, as the rev mob code is probably conditionally checking for it where required.
What you're probably doing is strong linking the library, whereas you should be weak linking it. Strong linking a library (the default) means it is marked as being required, and thus won't run on iOS 5 (as the library doesn't exist). Whereas when you weak link a library it is optional, and will happily run on older iOS versions (but you need to make sure you conditionally check for the library when you call any of its APIs).
To weak link a library, open up your project's Build Phases settings panel, and in the drop-down to the right of the AdSupport library change it from 'Required' to 'Optional'.