Extract QuickFixCmdPreMake and QuickFixCmdPostMake and return early for non-Bundler projects

In issue #21, @sigvei highlighted that `s:project()` is evaluated on "line 0"
of the `autocommand` *even when not in a Bundler project*. This `throw`s and
manifests as 'E488: Trailing characters'.

This call was changed in ff2a956 consistent with  @bpinto's comment.

Introducing and calling `s:QuickFixCmdPostMake()` prevents this evaluation.

    autocommand call s:QuickFixCmdPostMake()
    line 0: call s:QuickFixCmdPostMake()
    calling function <SNR>68_QuickFixCmdPostMake()
    line 1:   if &makeprg =~# '^bundle' && exists('b:bundler_root')
    line 2:     call s:pop_command()
    line 3:     call s:project().paths('refresh')
    line 4:   endif
    function <SNR>68_QuickFixCmdPostMake returning #0