Fix for issue Cleveroad#38

Changing ViewHolder visibility to public in order to support Kotlin applications

Author: Rubén Viguera Marañón

library/src/main/java/com/cleveroad/adaptivetablelayout/ViewHolder.java

@@ -6,7 +6,7 @@
 /**
  * A {@link ViewHolder} describes an item view and metadata about its place within the {@link AdaptiveTableLayout}.
  */
-interface ViewHolder {
+public interface ViewHolder {
     /**
      * @return item represents the item of the {@link AdaptiveTableLayout}
      */