Run Travis CI against both JDK 7 and 8.
Run Travis CI against both JDK 7 and 8.

Also, upgrade to Travis's container-based infrastructure.

Fixes #547

erwa/azkaban,sunghyuk/azkaban,davidzchen/azkaban,erwa/azkaban,logiclord/azkaban,erwa/azkaban,mradamlacey/azkaban,HappyRay/azkaban,mariacioffi/azkaban,mariacioffi/azkaban,azkaban/azkaban,erwa/azkaban,johnyu0520/azkaban,poporisil/azkaban3,reallocf/azkaban,HappyRay/azkaban,inoviaazkaban/azkaban,inoviaazkaban/azkaban,reallocf/azkaban,mradamlacey/azkaban,binhnv/azkaban,relateiq/azkaban,chengren311/azkaban,evlstyle/azkaban,poporisil/azkaban3,jackrjli/azkaban,davidzchen/azkaban,HappyRay/azkaban,azkaban/azkaban,chengren311/azkaban,inoviaazkaban/azkaban,jackrjli/azkaban,jackrjli/azkaban,binhnv/azkaban,johnyu0520/azkaban,evlstyle/azkaban,poporisil/azkaban3,mradamlacey/azkaban,reallocf/azkaban,azkaban/azkaban,logiclord/azkaban,researchgate/azkaban,evlstyle/azkaban,binhnv/azkaban,mariacioffi/azkaban,HappyRay/azkaban,sunghyuk/azkaban,azkaban/azkaban,HappyRay/azkaban,researchgate/azkaban,relateiq/azkaban,mradamlacey/azkaban,poporisil/azkaban3,davidzchen/azkaban,chengren311/azkaban,reallocf/azkaban,evlstyle/azkaban,logiclord/azkaban,relateiq/azkaban,researchgate/azkaban,chengren311/azkaban,sunghyuk/azkaban,mariacioffi/azkaban,johnyu0520/azkaban,mariacioffi/azkaban,sunghyuk/azkaban,jackrjli/azkaban,logiclord/azkaban,davidzchen/azkaban,erwa/azkaban,jackrjli/azkaban,binhnv/azkaban,poporisil/azkaban3,sunghyuk/azkaban,relateiq/azkaban,azkaban/azkaban,johnyu0520/azkaban,HappyRay/azkaban,reallocf/azkaban,azkaban/azkaban,binhnv/azkaban,mradamlacey/azkaban,researchgate/azkaban,chengren311/azkaban