Add travis support for push builds #805
Description
Feature Request
In PR #766, support for testing push builds was added, which allowed users to run travis CI on their own branches without having to modify their fork of the SDK. However, it caused all builds of travis to modify the Gopkg.toml file, even when they shouldn't have been. PR #793 reverted the change as it caused the tag builds to fail, as the tag builds were specifying both a version and a source+revision, which is a conflict in dep. PR #795 comments out the version line in the Gopkg.toml for PR builds which fixes issues testing new PRs in version locked branches (v0.1.x and v0.2.x), and also would fix this issue.
However, to make sure the Gopkg.toml file is correct, we should not modify builds happening on non-PR branches or on tag builds, as that could mask typos (for instance, branch v0.2.0 vs v0.2.x). We need to reimplement #766 in a way that allows developers/contributors to run CI testing on their repos while allowing the tests to run with an unmodified Gopkg.toml on non-PR builds for the main operator-sdk repo.