added openjdk8 to .travis.yml

[krichter722]

Short description of what this resolves:

Test might fail with OpenJDK while they pass with Oracle JDK.

Changes proposed in this pull request:

• Testing everything with OpenJDK8 that is tested with Oracle JDK 8 on Travis CI

[bartoszmajsak]

Thanks for your contribution. Makes me wonder if we really need tests on both JDKs (OpenJDK and Oracle). What would be the benefit @krichter722?

[krichter722]

Thanks for your contribution. Makes me wonder if we really need tests on both JDKs (OpenJDK and Oracle). What would be the benefit @krichter722?

I started investigating an issue and suspected it to be related to OpenJDK (strange stack in a GlassFish version I don't recall, but it's not important) and wanted to see whether devs are aware of eventual OpenJDK and Oracle JDK differences which might not influence arquillian-container-glassfish. However, testing both (in parallel without much slowdown) prevents unnecessary wondering. Now, I'm sharing this without thinking that there's a large number of use cases.