Configure travis to test against multiple jdks.
Configure travis to test against multiple jdks.

Testing against multiple jdks is offered by travis by default, so
we may as well. I'm currently investigating running it against gae
as well, but I'm not sure that travis supports running against that
environment.

zs-engg/stripe-sdk,rgarbi/stripe-java,stripe/stripe-java,kbouzidi/stripe-java,shalecraig/stripe-java,AdamStelmaszczyk/stripe-java,hardtoe/stripe-java