Fix Emacs-snapshot build

[geraldus]

Currently Emacs-snapshot Travis-CI build fails with following error:

In toplevel form:
haskell.el:90:63:Error: looking-back called with 1 argument, but requires 2-3
make: *** [haskell.elc] Error 1

According to documentaion of looking-back function in recent version
of Emacs, second and third arguments are optional. Second argument is
LIMIT, here is what documented:

LIMIT if non-nil speeds up the search by specifying a minimum
starting position, to avoid checking matches that would start
before LIMIT.

Thus, it turns that we can safely provide nil as LIMIT, which will not
affect current bahavoiur, but will fix error message.

Updated as suggested by @gracjan.

[gracjan]

@geraldus: Use (line-beginning-position). There is a reason they require that LIMIT argument and beginning of line is the correct thing in this case.