build images in 2.7 for python code in CC builds

Problem:
Build tools in CC repos use python 2.7 rather than 3.x, meaning that python code used in the build process fails when run in the 3.x build for CCCL nightly regressions.

Analysis:
Choice to use 3.x rather than 2.7 for "deploy" section was based on a slight speed advantage. Build can be moved back to 3.x when python code in CC repose is 3.x compatible.

Solution:
Deploy changed to 2.7

.travis.yml

@@ -33,12 +33,12 @@ deploy:
     skip_cleanup: true
     script: ./build-tools/system-test-img.sh F5Networks/marathon-bigip-ctlr $MARATHON_BIGIP_CTLR_COMMIT_ISH ./build-tools/build-runtime-images.sh $DOCKER_NAMESPACE
     on:
-      python: "3.5"
+      python: "2.7"
       all_branches: true
   # push k8s dev-image for nightly regression tests
   - provider: script
     skip_cleanup: true
     script: ./build-tools/system-test-img.sh F5Networks/k8s-bigip-ctlr $K8S_BIGIP_CTLR_COMMIT_ISH "make prod" $DOCKER_NAMESPACE
     on:
-      python: "3.5"
+      python: "2.7"
       all_branches: true