Add gcc 6 C++11 to travis builds.
Add gcc 6 C++11 to travis builds.

Aakash1312/appleseed,gospodnetic/appleseed,pjessesco/appleseed,gospodnetic/appleseed,aiivashchenko/appleseed,Vertexwahn/appleseed,luisbarrancos/appleseed,dictoon/appleseed,est77/appleseed,appleseedhq/appleseed,Aakash1312/appleseed,dictoon/appleseed,Vertexwahn/appleseed,aiivashchenko/appleseed,dictoon/appleseed,Biart95/appleseed,glebmish/appleseed,est77/appleseed,dictoon/appleseed,glebmish/appleseed,glebmish/appleseed,appleseedhq/appleseed,appleseedhq/appleseed,aiivashchenko/appleseed,pjessesco/appleseed,aytekaman/appleseed,Aakash1312/appleseed,aiivashchenko/appleseed,appleseedhq/appleseed,aytekaman/appleseed,pjessesco/appleseed,Aakash1312/appleseed,Biart95/appleseed,Vertexwahn/appleseed,Biart95/appleseed,glebmish/appleseed,est77/appleseed,Biart95/appleseed,luisbarrancos/appleseed,Aakash1312/appleseed,aytekaman/appleseed,est77/appleseed,aytekaman/appleseed,pjessesco/appleseed,est77/appleseed,gospodnetic/appleseed,aiivashchenko/appleseed,gospodnetic/appleseed,gospodnetic/appleseed,pjessesco/appleseed,Biart95/appleseed,Vertexwahn/appleseed,glebmish/appleseed,Vertexwahn/appleseed,aytekaman/appleseed,dictoon/appleseed,luisbarrancos/appleseed,luisbarrancos/appleseed,luisbarrancos/appleseed,appleseedhq/appleseed