If you are staying on 1.1.8, you just need to include the Mustache plugin and that's it. Your .mustache files will be parsed and rendered using Mustache and your .ejs files will work as before using EJS. If you are using script tags to store your templates, make sure they have the right type: text/ejs or text/mustache.
If you are upgrading to 2.x, Mustache is part of core and EJS is a plugin, so you would have to include the EJS plugin to make everything work.