Setting "Skip Install" to YES for OAuth

A fix for [Issue #416](https://github.com/forcedotcom/SalesforceMobileSDK-iOS/issues/416), where archiving apps seems to not work when libraries are set to install at install/archive time.  This shouldn't impact the library builds for the SDK, as we don't use the `install` target of xcodebuild.

NB: All of the other library projects are already set to skip install.