bump to 6.0 as there are backwards incompatible changes (#333)

Author: mr-c

Makefile

@@ -33,7 +33,7 @@ COVBASE=coverage run --branch --append --source=${MODULE} \
 
 # Updating the Major & Minor version below?
 # Don't forget to update setup.py as well
-VERSION=5.1.$(shell date +%Y%m%d%H%M%S --utc --date=`git log --first-parent \
+VERSION=6.0.$(shell date +%Y%m%d%H%M%S --utc --date=`git log --first-parent \
 	--max-count=1 --format=format:%cI`)
 
 ## all         : default task

setup.py

@@ -100,7 +100,7 @@
 
 setup(
     name="schema-salad",
-    version="5.1",  # update the VERSION prefix in the Makefile as well 🙂
+    version="6.0",  # update the VERSION prefix in the Makefile as well 🙂
     description="Schema Annotations for Linked Avro Data (SALAD)",
     long_description=open(README).read(),
     long_description_content_type="text/x-rst",