Podfile: update to new version

[ChristophTobler]

This resolves the error:

fatal error: 'Flutter/Flutter.h' file not found

which seems to occur with new versions. See flutter/flutter#16036 for more infos

I just created a new flutter project and copied the Podfile like explained in the issue.

For completeness, here's my flutter doctor output:

Doctor summary (to see all details, run flutter doctor -v):
[✓] Flutter (Channel beta, v0.9.4, on Mac OS X 10.14 18A389, locale en-CH)
[✓] Android toolchain - develop for Android devices (Android SDK 28.0.3)
[✓] iOS toolchain - develop for iOS devices (Xcode 10.1)
[✓] Android Studio (version 3.2)
    ✗ Flutter plugin not installed; this adds Flutter specific functionality.
    ✗ Dart plugin not installed; this adds Dart specific functionality.
[✓] VS Code (version 1.28.2)
[✓] Connected devices (1 available)

• No issues found!

[cheft]

copy Podfile from https://github.com/flutter/flutter/blob/master/packages/flutter_tools/templates/cocoapods/Podfile-objc