Travis updated to Xenial.
Travis updated to Xenial.

offa/seasocks,offa/seasocks,mattgodbolt/seasocks,mattgodbolt/seasocks,offa/seasocks,mattgodbolt/seasocks,offa/seasocks,mattgodbolt/seasocks,mattgodbolt/seasocks,mattgodbolt/seasocks,offa/seasocks,offa/seasocks