Update travis to use xenial as default dist.
Update travis to use xenial as default dist.

real-logic/Aeron,EvilMcJerkface/Aeron,mikeb01/Aeron,real-logic/Aeron,real-logic/Aeron,EvilMcJerkface/Aeron,mikeb01/Aeron,mikeb01/Aeron,EvilMcJerkface/Aeron,mikeb01/Aeron,EvilMcJerkface/Aeron,real-logic/Aeron