Make BikeCommonFalgEncoder abstract (graphhopper#941)

Flag-Encoders must call init() as their last action in the constructor. Therefore any FlagEncoder that does not call init() must be abstract.

Author: tobias74

core/src/main/java/com/graphhopper/routing/util/BikeCommonFlagEncoder.java

@@ -36,7 +36,7 @@
  * @author Nop
  * @author ratrun
  */
-public class BikeCommonFlagEncoder extends AbstractFlagEncoder {
+abstract public class BikeCommonFlagEncoder extends AbstractFlagEncoder {
     /**
      * Reports whether this edge is unpaved.
      */