Use standard VM Travis infrastructure (#272)
Use standard VM Travis infrastructure (#272)

Per https://blog.travis-ci.com/2018-10-04-combining-linux-infrastructures, the Travis container infrastructure is deprecated and will soon be disabled.  `sudo: false` opted us into it, so removing that line will put our builds back on the still-good VM infrastructure.
benjamin-bader/thrifty,benjamin-bader/thrifty,benjamin-bader/thrifty