disabled embree on travis linux, embree cmake is not willing to use the correct gcc

Former-commit-id: b7c97b04d27a403807a147fd95f28d256db53184

.travis.yml

@@ -17,7 +17,7 @@ matrix:
         - cd tutorial
         - mkdir build
         - cd build
-        - cmake -DLIBIGL_USE_STATIC_LIBRARY=ON -DCMAKE_CXX_COMPILER=g++-4.8 -DCMAKE_C_COMPILER=gcc-4.8 ../
+        - cmake -DLIBIGL_USE_STATIC_LIBRARY=ON -DCMAKE_CXX_COMPILER=g++-4.8 -DCMAKE_C_COMPILER=gcc-4.8 -DLIBIGL_WITH_EMBREE=OFF ../
         - make -j 2
       addons:
         apt: