Update travis-ci to build and push releases to DockerHub (#22)

* test python 3.7

* push all tagged releases to dockerhub with travis-ci

.travis.yml

@@ -1,5 +1,5 @@
-sudo: false
-dist: trusty
+sudo: true
+dist: xenial
 
 git:
   depth: false
@@ -10,6 +10,7 @@ python:
   - 2.7
   - 3.5
   - 3.6
+  - 3.7
 
 cache: pip
 
@@ -23,13 +24,14 @@ script:
 jobs:
   include:
     - stage: deploy
-      if: (branch = master OR branch =~ ^r[0-9]+.[0-9]+) AND type != pull_request
+      if: tag IS present
+      python: 3.6
       script:
       - echo "$DOCKER_PASSWORD" | docker login -u "$DOCKER_USERNAME" --password-stdin
       - docker build -t "$TRAVIS_REPO_SLUG" .
       - docker images
       - docker tag "$TRAVIS_REPO_SLUG" "$TRAVIS_REPO_SLUG":latest
-      - if [[ "$TRAVIS_BRANCH" != "master" ]] ; then docker tag "$TRAVIS_REPO_SLUG" "$TRAVIS_REPO_SLUG":"${TRAVIS_BRANCH#r}" ; fi
+      - docker tag "$TRAVIS_REPO_SLUG" "$TRAVIS_REPO_SLUG":"${TRAVIS_TAG}"
       - docker push "$TRAVIS_REPO_SLUG"
 
 after_success: