Update dependency react-native-linear-gradient to v2.5.6

Thalia Technicie
2019-11-14T13:59:49+01:00

This MR contains the following updates: | Package | Type | Update | Change | |---|---|---|---| | [react-native-linear-gradient](https://github.com/react-native-community/react-native-linear-gradient) | dependencies | patch | [`2.5.4` ->...