* Automatically build and push docker image to dashpay/dashd-develop
* Remove DOCKER_REPO env var from .travis.yml and add new-line to .gitignore
This should not have been there and results in errournous pushes to
"dashpay/dashd/dashpay/dashd-develop".
Also add missing new-line in .gitignore
* Fix Travis build failures happening due to large writes to stdout
Travis issue: https://github.com/travis-ci/travis-ci/issues/4704
Build failure happened since the introduction of secrets in .travis.yml
* Fix BUILD_DIR
Due to changes in develop the build dir has changed to be just "build".
* Remove bogus upload example from .travis.yml
* Switch to pushing on "develop" branch
Used "pr_docker" before for testing purposes.
* Only push docker image when TRAVIS_PULL_REQUEST = false
