[LICENSE] Add SPDX Identifier
diff --git a/.travis.yml b/.travis.yml
index 2083d54..10a17a9 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -11,6 +11,7 @@
 # WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 # See the License for the specific language governing permissions and
 # limitations under the License.
+# SPDX-License-Identifier: Apache-2.0
 
 language: minimal