3PL prior options not correct

[jplalor]

py-irt/py_irt/models/three_param_logistic.py

Line 36 in c2d8f73

raise ValueError("Options for priors are vague and hierarchical")

Only available prior for 3PL is hierarchical.

[charchit7]

Thanks for noticing the error, John!

[jplalor]

Fixed in #32