v1.3.5: Adds the document picker delegate for versions below iOS 11 (#87)

* If file not choosen getFile() should return null

* minor fix

* fixes an issue where sometimes getFilePath could return a List instead of a String

* minor fixes

* Update README.md

* Fix for background activities trying to register the plugin (#66)

* Beta (#55)

* If picker is canceled in `getFile()`, it should return `null`

* Beta (#60)

* Added a check to registerWith to prevent background flutter view from trying to register the plugin, as there won't be any activity.

* 1.3.3 > 1.3.4

* removes a build.gradle depdency that should't have been added in the last commit

* Adds the document picker delegate for versions below iOS 11