Update Travis tests for Py3 to run TF 1.6

PiperOrigin-RevId: 188563278

Author: Ryan Sepassi

.travis.yml

@@ -16,7 +16,7 @@ matrix:
     - python: "3.6"
       env: TF_VERSION="1.4.*"
     - python: "3.6"
-      env: TF_VERSION="1.6.*"
+      env: TF_VERSION="1.5.*"
 before_install:
   - echo "deb [arch=amd64] http://storage.googleapis.com/tensorflow-serving-apt stable tensorflow-model-server tensorflow-model-server-universal" | sudo tee /etc/apt/sources.list.d/tensorflow-serving.list
   - curl https://storage.googleapis.com/tensorflow-serving-apt/tensorflow-serving.release.pub.gpg | sudo apt-key add -