Remove circular dependency from Podspec

Part of the latest pre-release version of CocoaPods is a new dependency manager that isn't capable of handling circular dependencies in Podspecs. As a result, trying to install SlackTextViewController via the CocoaPods 0.35 RC results in an error. (see https://github.com/CocoaPods/CocoaPods/issues/2732)

This seems to fix the SlackTextViewController pod for me, at least when installing it as a development pod from a local path. A pod spec inherits all of its subspecs by default (see the [Podfile docs](http://guides.cocoapods.org/syntax/podspec.html#subspec), so explicitly listing the subspec as a dependency isn't necessary.

Cheers!