You might need to use a shim since Backbone according to the RequireJS expects Backbone to be in the base url directory. In your case, it appears to be in the /lib instead. Config Shim. Depending on the RequireJS version that you're using, you just have to make sure as well that the library that you're using is AMD compatible.
In your requireJS configuration:
shim: {
backbone: {
deps: ["underscore", "jquery"],
attach: "Backbone"
},
underscore: {
attach: "_"
}
}