Specify Eigen3 include path on macOS

* Orocos KDL depends upon Eigen 3.2, an older version than
    the Homebrew default, requiring that the EIGEN3_INCLUDE_DIR
    be specified to cmake providing the Eigen3 include path

.travis.yml

@@ -184,7 +184,7 @@ before_install:
           export INSTALLED_APP_PATH="/usr/local/MacOS/FreeCAD"
        fi
 
-       export CMAKE_ARGS="${CMAKE_OPTS} -DFREECAD_USE_EXTERNAL_KDL=ON"
+       export CMAKE_ARGS="${CMAKE_OPTS} -DFREECAD_USE_EXTERNAL_KDL=ON -DEIGEN3_INCLUDE_DIR=$(brew --prefix eigen@3.2)/include/eigen3"
        ;;
 
    *)