Update composer.json
Update composer.json
Added notice about SS 3.1
Added notice about SS 3.1
Changed require to 3.0.*

Issues have come to my attention about 3.1 and SortableGridField, so for the moment SortableGridField requires 3.0.x
Fixed compatibility issues with SilverStripe 3.1 (issue #19)
et 	Fixed compatibility issues with SilverStripe 3.1 (issue #19) 
Formatting fixes