Override hashCode getter

Overrides the hashCode getter to silence the warning range generated while building.
```
packages/range/range.dart:74:17: Hint: The class 'Range' overrides 'operator==', but not 'get hashCode'.
  bool operator ==(Range other) {
                ^^
```
Implemented according to https://www.dartlang.org/docs/dart-up-and-running/contents/ch03.html