Speed up core.prior classes

To solve issue #416 (closed)

bilby/core/prior.py

@@ -13,7 +13,7 @@ import scipy.stats
 from scipy.integrate import cumtrapz
 from scipy.interpolate import interp1d
 from scipy.special import erf, erfinv, xlogy, log1p,\
-    gamma, gammaln, gammainc, gammaincinv, stdtr, stdtrit
+    gammaln, gammainc, gammaincinv, stdtr, stdtrit
 from matplotlib.cbook import flatten
 
 # Keep import bilby statement, it is necessary for some eval() statements