Update Dockerfiles using ubuntu:vivid to xenial. #284

Job	Run time
get-matrix	4s
macos	0s
linux (IMPL=tcl)	43s
linux (IMPL=racket)	1m 14s
linux (IMPL=r)	1m 22s
linux (IMPL=python python_MODE=python3)	50s
linux (IMPL=python python_MODE=python2)	47s
linux (IMPL=ps)	49s
linux (IMPL=perl)	41s
linux (IMPL=objpascal)	47s
linux (IMPL=make NO_SELF_HOST=1)	1m 17s
linux (IMPL=io NO_SELF_HOST_PERF=1)	1m 20s
linux (IMPL=forth)	45s
linux (IMPL=common-lisp)	1m 43s
linux (IMPL=bash NO_SELF_HOST=1)	58s
linux (IMPL=awk)	41s
linux (IMPL=ada)	1m 11s
	15m 12s

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