travis: Treat warnings as errors

.travis.sh

@@ -1,6 +1,6 @@
 function generate()
 {
-    cmake -G "Unix Makefiles" -DASSIMP_NO_EXPORT=$TRAVIS_NO_EXPORT -DBUILD_SHARED_LIBS=$SHARED_BUILD -DASSIMP_COVERALLS=$ENABLE_COVERALLS
+    cmake -G "Unix Makefiles" -DASSIMP_NO_EXPORT=$TRAVIS_NO_EXPORT -DBUILD_SHARED_LIBS=$SHARED_BUILD -DASSIMP_COVERALLS=$ENABLE_COVERALLS -DASSIMP_ERROR=ON
 }
 
 if [ $ANDROID ]; then