Add allow_failures and fast_finish.

.travis.yml

@@ -29,6 +29,19 @@ env:
     - RACKET_VERSION=6.1.1
     - 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
+
+# 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.
+matrix:
+  fast_finish: true
+
+
 before_install:
   - git clone https://github.com/greghendershott/travis-racket.git
   - cat travis-racket/install-racket.sh | bash # pipe to bash not sh!