I'm in contact with the author of DragonTravel and we are discussing the license and which changes we can do. Next version will be already with the correct License agreements etc.
Version: 1.3.5
[20:22:20 ERROR]: null
org.bukkit.command.CommandException: Unhandled exception executing command 'dragonride' in plugin DragonRide v1.3.5
at org.bukkit.command.PluginCommand.execute(PluginCommand.java:46) ~[spigot1.8.8.jar:git-Spigot-21fe707-e1ebe52]
at org.bukkit.command.SimpleCommandMap.dispatch(SimpleCommandMap.java:141) ~[spigot1.8.8.jar:git-Spigot-21fe707-e1ebe52]
at org.bukkit.craftbukkit.v1_8_R3.CraftServer.dispatchCommand(CraftServer.java:641) ~[spigot1.8.8.jar:git-Spigot-21fe707-e1ebe52]
at net.minecraft.server.v1_8_R3.PlayerConnection.handleCommand(PlayerConnection.java:1162) [spigot1.8.8.jar:git-Spigot-21fe707-e1ebe52]
at net.minecraft.server.v1_8_R3.PlayerConnection.a(PlayerConnection.java:997) [spigot1.8.8.jar:git-Spigot-21fe707-e1ebe52]
at net.minecraft.server.v1_8_R3.PacketPlayInChat.a(PacketPlayInChat.java:45) [spigot1.8.8.jar:git-Spigot-21fe707-e1ebe52]
at net.minecraft.server.v1_8_R3.PacketPlayInChat.a(PacketPlayInChat.java:1) [spigot1.8.8.jar:git-Spigot-21fe707-e1ebe52]
at net.minecraft.server.v1_8_R3.PlayerConnectionUtils$1.run(SourceFile:13) [spigot1.8.8.jar:git-Spigot-21fe707-e1ebe52]
at java.util.concurrent.Executors$RunnableAdapter.call(Unknown Source) [?:1.8.0_281]
at java.util.concurrent.FutureTask.run(Unknown Source) [?:1.8.0_281]
at net.minecraft.server.v1_8_R3.SystemUtils.a(SourceFile:44) [spigot1.8.8.jar:git-Spigot-21fe707-e1ebe52]
at net.minecraft.server.v1_8_R3.MinecraftServer.B(MinecraftServer.java:715) [spigot1.8.8.jar:git-Spigot-21fe707-e1ebe52]
at net.minecraft.server.v1_8_R3.DedicatedServer.B(DedicatedServer.java:374) [spigot1.8.8.jar:git-Spigot-21fe707-e1ebe52]
at net.minecraft.server.v1_8_R3.MinecraftServer.A(MinecraftServer.java:654) [spigot1.8.8.jar:git-Spigot-21fe707-e1ebe52]
at net.minecraft.server.v1_8_R3.MinecraftServer.run(MinecraftServer.java:557) [spigot1.8.8.jar:git-Spigot-21fe707-e1ebe52]
at java.lang.Thread.run(Unknown Source) [?:1.8.0_281]
Caused by: java.lang.NoSuchMethodError: org.bukkit.entity.Player.getPassengers()Ljava/util/List;
at de.sirzontax.dragonride.DragonManager.dismount(DragonManager.java:44) ~[?:?]
at de.sirzontax.dragonride.core.commands.DragonRideTabCommand.onCommand(DragonRideTabCommand.java:49) ~[?:?]
at org.bukkit.command.PluginCommand.execute(PluginCommand.java:44) ~[spigot1.8.8.jar:git-Spigot-21fe707-e1ebe52]
... 15 more