Revert "Disable bootstrapping by default."
    
    This doesn't work well since non-bootstrapping cannot be used to move
    from one minor release to the next.  For example if you have OCaml
    4.07.0 then you cannot use non-bootstrapping to move to 4.07.1,
    because the *.cmi files are incompatible between the two releases.
    
    We need to rethink this.
    
    This reverts commit 8f288a07f3f14d0415138c013f46e082c83cfcdf.