Standardize version references to "PROPKA 3".

Partially addresses https://github.com/jensengroup/propka-3.1/pull/61#pullrequestreview-433445231

README.md

@@ -1,7 +1,7 @@
-# PROPKA 3.2
+# PROPKA 3
 
 PROPKA predicts the pKa values of ionizable groups in proteins
-(version 3.0) and protein-ligand complexes (version 3.2 and later)
+(version 3.0) and protein-ligand complexes (version 3.1 and later)
 based on the 3D structure.
 
 For proteins without ligands, both version should produce the same result.

propka/__init__.py

@@ -1,4 +1,4 @@
-"""PROPKA 3.2
+"""PROPKA 3
 
 See https://github.com/jensengroup/propka-3.1 for more information.