Deployment will be handled by ASF's Jenkins instances which trigger
the deployment of snapshot versions every night. This way, we have not
only separated deployment from CI testing, but also improved the
reliability of the test and deployment infrastructure.

- remove Travis dependency from Maven deploy script

- set force-dependency version to Flink version

This assures deployment works when only credentials for the snapshot
deployment are available. Previously, we deployed snapshots with release
credentials. These credentials would be over-privileged.

- remove old credentials from Travis config file

This closes #1619.