Update .travis.yml

Add 6.2.1 and allow HEAD failures

.travis.yml

@@ -12,13 +12,14 @@ env:
     - RACKET_VERSION=6.0
     - RACKET_VERSION=6.1
     - RACKET_VERSION=6.2
+    - RACKET_VERSION=6.2.1
     - 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.