Throw exception if points are equal in lineSlice

[cammace]

When using TurfMisc.lineSlice and passing in identical points, an exception should be thrown.

https://github.com/mapbox/mapbox-java/blob/master/mapbox/libjava-services/src/main/java/com/mapbox/services/api/utils/turf/TurfMisc.java#L52

[cammace]

#513 adds an exception. Closing.