Update unification library

Author: brandonwillard

requirements.txt

@@ -6,7 +6,7 @@ tf-nightly-2.0-preview==2.0.0.dev20191002
 tensorflow-estimator-2.0-preview>=1.14.0.dev2019090801
 pymc3>=3.6
 multipledispatch>=0.6.0
-unification>=0.2.2
+logical-unification>=0.2.2
 kanren @ git+https://github.com/pymc-devs/kanren.git@symbolic-pymc#egg=kanren-0.2.3
 cons>=0.1.3
 toolz>=0.9.0