BUG: Work-around a numpy regression affecting pandas.eval() with numexpr

numpy 1.7.0 erroneously raises IndexError instead of ValueError
from ndarray.item() when the array is not of length 1. This can be
seen as a failure of

    computation.tests.test_eval.TestScope.test_global_scope

for the cases that engine='numexpr'.

Absorb the splatter from this regression by explicitly catching the
erroneous IndexError.

closes #5535.