CI tests fail sporadically on Travis

CI tests have been failing sporadically lately, more often than not. I'm pretty sure this is caused by the kernel's OOM killer, given this line in the log:

16/04/29 23:57:37.254 ContextLauncher-1: Child process exited with code 137.^M

That's a SIGKILL. We can look at a few things, including:

• lower memory settings for the test processes (currently -Xmx4g)
• lower memory settings for launched Spark processes (currently the default, 1g per process)
• switch to a larger env (see https://docs.travis-ci.com/user/ci-environment/, trusty provides ~7G).