(MAINT) Indicate supported PostgreSQL versions

Prior to this commit, there was an old limitation noted that we only support up to PostgreSQL 9.5.  

After this commit, we remove that limitation because we support whatever the default for each OS is.

Author: npwalker

README.md

@@ -320,7 +320,7 @@ For information on the classes and types, see the [REFERENCE.md](https://github.
 
 ## Limitations
 
-Works with versions of PostgreSQL from 8.1 through 9.5.
+Works with versions of PostgreSQL on supported OSes.  
 
 For an extensive list of supported operating systems, see [metadata.json](https://github.com/puppetlabs/puppetlabs-postgresql/blob/master/metadata.json)