Update .travis.yml

.travis.yml

@@ -20,13 +20,13 @@ env:
     - RACKET_VERSION=6.8
     - RACKET_VERSION=6.9
     - RACKET_VERSION=6.10
-    - RACKET_VERSION=HEAD
+    # - RACKET_VERSION=HEAD
 
 # You may want to test against certain versions of Racket, without
 # having them count against the overall success/failure.
 matrix:
   allow_failures:
-    - env: RACKET_VERSION=HEAD
+    # - env: RACKET_VERSION=HEAD
   # Fast finish: Overall build result is determined as soon as any of
   # its rows have failed, or, all of its rows that aren't allowed to
   # fail have succeeded.