Run tests on ubuntu with xvfb (#179) #369

Job	Run time
test (3.12)	1m 16s
test (3.14)	1m 13s
test (3.13)	1m 12s
test (3.11)	1m 20s
test (3.10)	1m 25s
Indicate completion to coveralls.io	13s
	6m 39s

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