everytime i try to put my minecraft on 1.7.9 or less it crashes so i can play on servers i want to play on

Put the summary of the bug you're having here

What I expected to happen was...:
to play 1.7.9 version but it crashed

What actually happened was...:
I wanted to play on the server called the nexus so I had to change my minecraft from 1.8.1 to 1.7.9 but when I tried to join my minecraft crashed

Steps to Reproduce:
1. Put a step by step guide on how to trigger the bug here
2. first I changed my minecraft to 1.7.9 version
3. then I tried to go on a 1.7.9 version server and then my minecraft crashed