roll back Sympy to 1.1.1 due to Kwant compatibility

science/environment.yml

@@ -10,7 +10,7 @@ dependencies:
   - conda-forge::numpy=1.15.1
 
   - conda-forge::kwant=1.3.3
-  - conda-forge::sympy=1.2.0
+  - conda-forge::sympy=1.1.1
   - conda-forge::adaptive=0.5.0
 
   # data analysis, visulation and serialisation