add redirect for new branch (#20)

Author: terakilobyte

config/redirects

@@ -1,8 +1,8 @@
 define: prefix docs/ruby-driver
 define: base https://www.mongodb.com/${prefix}
 define: mongoid_base https://www.mongodb.com/docs/mongoid
-define: versions v1.x v2.0 v2.1 v2.2 v2.3 v2.4 v2.5 v2.6 v2.7 v2.8 v2.9 v2.10 v2.11 v2.12 v2.13 v2.14 v2.15 v2.16 v2.17 master
-symlink: current -> v2.17
+define: versions v1.x v2.0 v2.1 v2.2 v2.3 v2.4 v2.5 v2.6 v2.7 v2.8 v2.9 v2.10 v2.11 v2.12 v2.13 v2.14 v2.15 v2.16 v2.17 v2.18 master
+symlink: current -> v2.18
 
 
 raw: /${prefix} -> ${base}/current/