Bump version for 2.3.1

Add release notes for 2.3.1 as well

CHANGES.txt

 Changelog
 =========
+2.3.1 (2017-01-31)
+------------------
+
+Bugs:
+
+* Fix regression in detection of E302 and E306; #618, #620
+
 2.3.0 (2017-01-30)
 ------------------

pycodestyle.py

@@ -66,7 +66,7 @@ try:
 except ImportError:
     from ConfigParser import RawConfigParser
-__version__ = '2.3.0'
+__version__ = '2.3.1'
 DEFAULT_EXCLUDE = '.svn,CVS,.bzr,.hg,.git,__pycache__,.tox'
 DEFAULT_IGNORE = 'E121,E123,E126,E226,E24,E704,W503'