travis: flip to distro xenial, drop sudo false

travis: flip to distro xenial, drop sudo false

The latter is the default these days and Travis will be removing sudo
soonish.

Flipping to xenial, allows us to remove a bunch of hacks we have. Plus
it prevents us from adding new ones, to workaround what seems like a
gcc/binutils bug. For example (from the upcoming meson build):

FAILED: ccache c++  -o src/gallium/targets/pipe-loader/pipe_r600.so ...
  ... src/util/libmesa_util.a ... /usr/lib/x86_64-linux-gnu/libz.so ...

src/util/libmesa_util.a(disk_cache.c.o): In function `deflate_and_write_to_disk':
_build/../src/util/disk_cache.c:746: undefined reference to `deflateInit_'
_build/../src/util/disk_cache.c:765: undefined reference to `deflate'
...

As we can see, even though libz.so is explicitly passed after the
object that requires it - the linker still fails to see the symbols.
Avoid all those situations - flip the switch.

Signed-off-by: Emil Velikov <emil.velikov@collabora.com>
Reviewed-by: Dylan Baker <dylan@pnwbakers.com>