Support ruby upgrades

Remove previous definition from bashrc when new ruby installed
so that new ruby is used.

Previously:

 # Generated by chef
 export PATH=$HOME/1.9.3-p362/bin:$PATH
 # Generated by chef
 export PATH=$HOME/1.9.3-p247/bin:$PATH

It lead to using old ruby anyway.

Now:

 # Generated by chef
 export PATH=$HOME/1.9.3-p362/bin:$PATH

Profit!