Make compiling working in ios app

[westoque]

Initial compiliation fails on ios with:

Xcode's output:
↳
    While building module 'FirebaseDatabase' imported from /Users/westoque/AndroidStudioProjects/happybit/ios/Pods/Headers/Public/Firebase/Firebase.h:26:
    In file included from <module-includes>:1:
    In file included from /Users/westoque/AndroidStudioProjects/happybit/ios/Pods/Target Support Files/FirebaseDatabase/FirebaseDatabase-umbrella.h:13:
    /Users/westoque/AndroidStudioProjects/happybit/ios/Pods/FirebaseDatabase/Firebase/Database/Public/FIRDatabase.h:154:1: error: property with 'retain (or strong)' attribute must be of object type
    @property (nonatomic, strong) dispatch_queue_t callbackQueue;
    ^
    1 error generated.
    In file included from /Users/westoque/GitHub/flutter/plugins/packages/cloud_functions/ios/Classes/CloudFunctionsPlugin.m:8:
    /Users/westoque/AndroidStudioProjects/happybit/ios/Pods/Headers/Public/Firebase/Firebase.h:26:13: fatal error: could not build module 'FirebaseDatabase'
        #import <FirebaseDatabase/FirebaseDatabase.h>
         ~~~~~~~^
    2 errors generated.
Encountered error while building for device.

This PR makes compiling working in app by requiring 'Firebase/Database' and setting ios.deployment_target to '6.0'

[pulyaevskiy]

Just an FYI: there is similar PR here #780 with some discussion around the issue.