First, if this is a fairly new project, I would recommend using the native REST functionality: the REST services module should not be neccesary anymore in Mendix 7.
To solve your actual error, you should download the REST Services module from the app store. That contains the necessary libraries, and should resolve your errors. Although the generally accepted practice in the Mendix community is to never change app store content, since this prevents updating the module, you may check if there have been no changes made to the module (right click the module in the project explorer, select Show history).
Mendix by default does not use a tool for dependency management.