Update .travis.yml

Author: spdegabrielle

.travis.yml

@@ -16,10 +16,7 @@ env:
     # Supply more than one RACKET_VERSION (as in the example below) to
     # create a Travis-CI build matrix to test against multiple Racket
     # versions.
-    - RACKET_VERSION=6.12
-    - RACKET_VERSION=7.0
-    - RACKET_VERSION=7.1
-    - RACKET_VERSION=7.2
+    - RACKET_VERSION=8.8
     - RACKET_VERSION=HEAD
 
 matrix: