bump version 1.8.1rc1

.bumpversion.cfg

@@ -1,5 +1,5 @@
 [bumpversion]
-current_version = 1.8.0
+current_version = 1.8.1rc1
 parse = (?P<major>[\d]+) # major version number
 	\.(?P<minor>[\d]+) # minor version number
 	\.(?P<patch>[\d]+) # patch version number