Tags are used to mark specific versions of ACGL. The master branch should be stable during semesters and the API defining version per semester is tagged in git (ws13, ss14 etc.).
Between Tag ws13 and ss14 GLM was updated to 0.9.5.2 which removed glm::translate( float, float, float ) (and similar). This now has to be done with a glm::vec3(): glm::translate( glm::vec3(float, float, float) ).