Add RTDB-emulator-based integration testing. (#316)

* Add RTDB-emulator-based integration testing.

* Combine unit tests and integration tests into a single job.

Author: yuchenshi

.travis.yml

@@ -5,10 +5,20 @@ python:
   - "3.5"
   - "3.6"
   - "pypy3.5"
-# command to install dependencies
-install: "pip install -r requirements.txt"
-before_script: 
-  - export PY_VERSION=`python -c 'import sys; print sys.version_info.major'`
-  - if [[ "$PY_VERSION" == '2' ]]; then ./lint.sh all; fi
-# command to run tests
-script: pytest
+
+jobs:
+  include:
+    - name: "Lint"
+      python: "2.7"
+      script: ./lint.sh all
+
+before_install:
+  - nvm install 8 && npm install -g firebase-tools
+script:
+  - pytest
+  - firebase emulators:exec --only database --project fake-project-id 'pytest integration/test_db.py'
+cache:
+  pip: true
+  npm: true
+  directories:
+  - $HOME/.cache/firebase/emulators