fix MvNormal error (#2464)

* fix MvNormal error

(cherry picked from commit b763b08)

* add import to MvNormal directly

Author: ferrine

pymc3/distributions/multivariate.py

@@ -10,7 +10,7 @@
 from scipy import stats, linalg
 
 from theano.tensor.nlinalg import det, matrix_inverse, trace
-
+import theano.tensor.slinalg
 import pymc3 as pm
 
 from pymc3.math import tround