[Epiphany] "I feel lucky" preference on Address bar

[Gavrila <gavrilus tiscalinet it>]

I'd like to disable the "I feel lucky" google search entering a term in
the address bar and pressing enter, replacing it with the standard
google search, but though I change the pref in pref.js in my home dir, I
can't rid of that feature. 
The file pref.js is always overwritten by epiphany itself: what should I
do? 

TIA