Update to allow intl in pubspec.yaml to be imported <= version 0.17.0…

[ebelliveau]

This is to enable compatibility within any apps requiring both firebase_auth^1.0.0 and charts_flutter when packaging the two components in the same app.

For example, when building a project which depends on the latest charts_flutter^0.9.0 and firebase_auth^1.0.0:

PS C:\Users\SomeUser\Development\someApp> flutter pub get
Because no versions of charts_flutter match >0.9.0 <0.10.0 and charts_flutter 0.9.0 depends on intl >=0.15.2 <0.17.0, charts_flutter ^0.9.0 requires intl >=0.15.2 <0.17.0.
And because firebase_auth >=1.0.0 depends on firebase_auth_web ^1.0.0 which depends on intl ^0.17.0, charts_flutter ^0.9.0 is incompatible with firebase_auth >=1.0.0.
So, because someApp depends on both firebase_auth ^1.0.0 and charts_flutter ^0.9.0, version solving failed.
Running "flutter pub get" in someApp...
pub get failed (1; So, because someApp depends on both firebase_auth ^1.0.0 and charts_flutter ^0.9.0, version solving failed.)

It's just a single comparator in the version designation (<= vs <), and the tests continute to pass, so I hope this is just a small omission/typo in version pinning between two (Google) libraries.

Closes #602

[ebelliveau]

Ah no - I just saw the other PR to address this. Closing as there's already discussion for a few days in the other thread.