Remove deprecated sudo: false.

Per https://blog.travis-ci.com/2018-11-19-required-linux-infrastructure-migration
and https://blog.travis-ci.com/2018-10-04-combining-linux-infrastructures,
Travis CI is eliminating its container-based infrastructure
and deprecating the `sudo` keyword.

Also add 7.1 to build matrix.

.travis.yml

@@ -1,10 +1,5 @@
 language: c
 
-# To use Travis CI's newer container infrastructure, use the following
-# line. (Also be sure RACKET_DIR is set to somewhere like ~/racket
-# that doesn't require sudo.)
-sudo: false
-
 env:
   global:
     # Supply a global RACKET_DIR environment variable. This is where