Unpin sphinx. cc: omnia-md/conda-recipes#909

Author: jchodera

openmm/meta.yaml

@@ -33,7 +33,7 @@ requirements:
     # on osx, need to install doxygen manually
     - doxygen   [not osx]
     # for building docs
-    - sphinx ==1.5.6
+    - sphinx >=1.5.6
     - sphinxcontrib-bibtex
     - sphinxcontrib-lunrsearch >=0.3
     - sphinxcontrib-autodoc_doxygen