Update SBTSubProjects.md

changed the method name from depends to dependsOn as per the github question http://stackoverflow.com/questions/21730323/play-framework-2-2-1-adding-non-play-java-project-as-sub-project, was facing the same issue and this solved it.
Merge pull request #2589 from rishikhare/patch-1

Update SBTSubProjects.md