Update .travis.yml to use openjdk8 from openjdk7

[mbernier]

Issue Summary

We had success with not using the deprecated openjdk7 in our sendgrid/java-http-client repo. I would have made this change:

jdk:
- oraclejdk8
- openjdk8

But I am not familiar enough with TravisCI to understand what this is:

matrix:
  include:
    - os: linux
      dist: precise
      jdk: openjdk7
    - os: linux
      jdk: oraclejdk8

My guess is that I could just replace jdk: but I would rather someone who knows wtf they are doing to help with this!

[childish-sambino]

Fixed by #103