Put a 1.8 Spigot/CraftBukkit JAR with filename craftbukkit-1.8-R0.1-SNAPSHOT.jar in lib/org/bukkit/craftbukkit/1.8-R0.1-SNAPSHOT.
craftbukkit-1.8-R0.1-SNAPSHOT.jar
lib/org/bukkit/craftbukkit/1.8-R0.1-SNAPSHOT