feat(nightlies): Add ability to build from nightlies (#211)

This patch adds the optional IMAGE env variable to set the base
image and adds nightly builds to Travis CI.

.travis.yml

@@ -1,6 +1,10 @@
 language: bash
 services: docker
 
+env:
+  - IMAGE=sentry:9.1
+  - IMAGE=getsentry/sentry:git
+
 script:
   - ./install.sh
   - docker-compose run --rm web createuser --superuser --email test@sentry.io --password test123TEST