Fixing Google Mobile Ads SDK 6.9.2 Podspec

Some changes were made with 6.9.2 that weren't accounted for with the previous submit:

- The version specific zip should be used.
- Directory name for source files changed.
- SDK now only supports iOS 5.0 and up.
- Not sure how "requires_arc" is interpreted, but this SDK now uses ARC's zeroing weak references so I bumped this to yes.