Setup a Jenkins server.
It can build each maven module as a separate project and then automatically rebuild any other projects that has a snapshot dependency relationship.
Update
The GIT plugin for Jenkins has a feature for managing the automatic merging of development feature branches into a "master" or "integration" branch. See the documentation:
https://wiki.jenkins-ci.org/display/JENKINS/Git+Plugin#GitPlugin-AdvancedFeatures
Another option is to control feature development using a code review tool like Gerrit. Again it's Gerrit that controls changes integrated onto the shared code stream and Gerrit can be integrated with Jenkins to ensuring all submitted change-sets pass the code tests.
Finally releases are cut using the M2 Release plugin, which is a wrapper around the standard release plugin. (Means you'll always have two types of build in Jenkins. Automated builds triggered by code commits and release builds explicitly triggered via the UI).