Podfile in readme has invalid syntax

[lispingit]

In https://github.com/Factual/segment-analytics-factual-engine-ios#cocoapods the podfile matches single quotes with double quotes. As such, I got a syntax error when I tried to use it in a file.

edit: I am referring to 'Analytics" and 'FactualEngineSDK"

[alfredp]

Updated the README

Version 4.9.0 is not yet uploaded to Bintray, and the podspec repo is not updated, so this podfile won't work just yet. The latest in repo is 4.7.0, and that won't work either as I'm now using the new version method available in 4.9.0