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
Merge pull request #1838 from jrioux/printing

Fix fold_frac_powers=True latex printing.