docs: Bump version to 1.8.

docs/conf.py

@@ -91,9 +91,9 @@ copyright = '2014-2016, Damien P. George and contributors'
 # built documents.
 #
 # The short X.Y version.
-version = '1.7'
+version = '1.8'
 # The full version, including alpha/beta/rc tags.
-release = '1.7'
+release = '1.8'
 
 # The language for content autogenerated by Sphinx. Refer to documentation
 # for a list of supported languages.