Extract QuickFixCmdPreMake and QuickFixCmdPostMake

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
Robust checking for Bundler project

Prior to Vim version 7.3.097, Using `:call` inside `if 0` does not see that a
function returns a `dict` and throws an error for `.` as if it were string
concatenation.

This was fixed in [revision faf4b09c396e](https://code.google.com/p/vim/source/detail?r=faf4b09c396e#).

This change works around that bug with an early `return`.
Merge pull request #24 from johnsyweb/syntastic-harmony

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