Updates pod spec with dependency on PLCrashReporter pod spec hosted on https://github.com/wherecloud/PLCrashReporter

I submitted the pod spec to Plausible Labs for them to handle the cocoapods integration of PLCrashReporter by themselves.
Will be able to submit the Ballista pod as soon as they shipped their pod or we'll ship the PLCrashReporter pod from our repository if they are not interested in doing this.