Update roundcube to version 1.3.1

docs/third_party-roundcube.md

@@ -1,9 +1,9 @@
-Download Roundcube 1.3.0 (26 June 2017, Roundcube Webmail 1.3.0 released) to the web htdocs directory and extract it (here `rc/`):
+Download Roundcube 1.3.1 (04 September 2017, Roundcube Webmail 1.3.1 released) to the web htdocs directory and extract it (here `rc/`):
 ```
 cd data/web/rc
-wget -O - https://github.com/roundcube/roundcubemail/releases/download/1.3.0/roundcubemail-1.3.0-complete.tar.gz | tar xfvz -
+wget -O - https://github.com/roundcube/roundcubemail/releases/download/1.3.1/roundcubemail-1.3.1-complete.tar.gz | tar xfvz -
 # Change folder name
-mv roundcubemail-1.3.0 rc
+mv roundcubemail-1.3.1 rc
 # Change permissions
 chown -R root: rc/
 ```