Loosen momentjs dependency

Version `~2.6.0` means `>= 2.6.0 < 2.7.0` which is too strict I think
(see [2.7.0 Changlelog](https://github.com/moment/moment/blob/develop/CHANGELOG.md#270-see-changelog)).

Version `^2.6.0` means `>= 2.6.0 < 3.0.0`.