Use openjdk8 for travis-ci macOS build

[j4ckofalltrades]

@steve-todorov

I've fixed the failing integration tests in #1958 by replacing the remote url from repo.spring.io to jcenter.bintray.com as recommended by @carlspring.

Travis CI build: https://travis-ci.org/github/j4ckofalltrades/strongbox/jobs/743752372

Pull Request Description

This pull request closes #1956
This pull request also closes #1958

Acceptance Test

• Building the code with mvn clean install -Dintegration.tests still works.
• Running mvn spring-boot:run in the strongbox-web-core still starts up the application correctly.
• Building the code and running the strongbox-distribution from a zip or tar.gz still works.
• The tests in the strongbox-web-integration-tests still run properly.

Questions

• Does this pull request break backward compatibility?

  • Yes
  • No

• Does this pull request require other pull requests to be merged first?

  • Yes, please see #...
  • No

• Does this require an update of the documentation?

  • Yes, please see strongbox/strongbox-docs#{PR_NUMBER}
  • No

Code Review And Pre-Merge Checklist

• My code follows the coding convention of this project.
• I have performed a self-review of my own code.
• I have commented my code in hard-to-understand areas.
• My changes generate no new warnings.
• I have added tests that prove my fix is effective or that my feature works.
• New and existing unit tests pass locally with my changes.

[carlspring]

LGTM! Thanks for these fixes! :)

[carlspring]

@j4ckofalltrades ,

The build seems to still be failing under Travis. Would you mind having another look? Thanks! :)

[steve-todorov]

LGTM! Thanks for spending the time to fix this!