Fix fold_frac_powers=True latex printing.

The latex printing of (x + 1)**Rational(3, 2) with fold_frac_powers=True was missing the opening and closing brackets. The latex printing of functions raised to rational powers is made consistent with the fold_frac_powers=False option.

Fixes issue 3670.
http://code.google.com/p/sympy/issues/detail?id=3670