travis-ci: fix unittest executor.

.travis.sh

@@ -6,7 +6,7 @@ function generate()
 if [ $ANDROID ]; then
     ant -v -Dmy.dir=${TRAVIS_BUILD_DIR} -f ${TRAVIS_BUILD_DIR}/port/jassimp/build.xml ndk-jni
 fi
-if [ $LINUX ]; then
+if [ "$TRAVIS_OS_NAME" = "linux" ]; then
     generate \
     && make -j4 \
     && sudo make install \