Crashing on 1.7.10

Put the summary of the bug you're having here
I created a server for me and my friends, and we wanted to use bukkit to use plugins. I switched the server type to bukkit and it changed it to 1.7.10, and my minecraft crashed. This happened to me on every server that I tried to join afterwards.
What I expected to happen was...:
Describe what you thought should happen here
What i wxpected to happen was to join the server regularly
What actually happened was...:
Describe what happened here
What actually happened was it crashed every time I attempted to join any server
Steps to Reproduce:
1. I edited my profile, and changed it 1.7.10
2. I clicked play
3. I tried to join my server, and it crashes.