Fix travis cmake call to enable coberalls.

.travis.sh

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