Bump version to 1.20

The next version would have been 1.17, but a bump to 1.20 was requested
per #67 to fix an issue with versioning of the Debian package

#67

Author: stefankoegl

jsonpatch.py

@@ -58,7 +58,7 @@
 
 # Will be parsed by setup.py to determine package metadata
 __author__ = 'Stefan Kögl <[email protected]>'
-__version__ = '1.16'
+__version__ = '1.20'
 __website__ = 'https://github.com/stefankoegl/python-json-patch'
 __license__ = 'Modified BSD License'