Bump SDK version

[dpasqualin]

Hi, I'm using the master channel on flutter (SDK 2.1), and when I run flutter packages get I receive the following error:

Because flutter_calendar 0.0.4 depends on date_utils any which requires SDK version >=1.20.1 <2.0.0, flutter_calendar 0.0.4 is forbidden. 

I've forked your projects (date_utils and flutter_calendar), changed the sdk requirement from <2.0.0 to <3.0.0 and apparently it still works good. I'm just not sure if I've made all necessary tests, do perhaps you could run the tests and update the package?

Thanks.

[ericwindmill]

This is fixed.