Bump the default build image to Ubuntu 16.
Bump the default build image to Ubuntu 16.

Also update the linux headers to the appropriate kernel version.
And pin GCC to 4.8 (instead of defaulting to 5.3).

mnaboka/dcos,mnaboka/dcos,mesosphere-mergebot/mergebot-test-dcos,dcos/dcos,GoelDeepak/dcos,vishnu2kmohan/dcos,amitaekbote/dcos,mellenburg/dcos,amitaekbote/dcos,dcos/dcos,kensipe/dcos,mesosphere-mergebot/mergebot-test-dcos,darkonie/dcos,mellenburg/dcos,surdy/dcos,xinxian0458/dcos,darkonie/dcos,mesosphere-mergebot/dcos,mesosphere-mergebot/mergebot-test-dcos,mesosphere-mergebot/dcos,asridharan/dcos,dcos/dcos,xinxian0458/dcos,branden/dcos,mnaboka/dcos,darkonie/dcos,lingmann/dcos,asridharan/dcos,dcos/dcos,mellenburg/dcos,GoelDeepak/dcos,mesosphere-mergebot/mergebot-test-dcos,kensipe/dcos,surdy/dcos,vishnu2kmohan/dcos,xinxian0458/dcos,jeid64/dcos,GoelDeepak/dcos,branden/dcos,lingmann/dcos,lingmann/dcos,darkonie/dcos,kensipe/dcos,lingmann/dcos,mnaboka/dcos,kensipe/dcos,surdy/dcos,BenWhitehead/dcos,BenWhitehead/dcos,darkonie/dcos,branden/dcos,mesosphere-mergebot/dcos,asridharan/dcos,vishnu2kmohan/dcos,mnaboka/dcos,asridharan/dcos,jeid64/dcos,jeid64/dcos,GoelDeepak/dcos,mesosphere-mergebot/dcos,jeid64/dcos,amitaekbote/dcos,mellenburg/dcos,dcos/dcos,amitaekbote/dcos,vishnu2kmohan/dcos,branden/dcos,surdy/dcos,BenWhitehead/dcos,BenWhitehead/dcos,xinxian0458/dcos