finetuning the build matrix

.travis.yml

@@ -7,6 +7,13 @@ env:
     - RACKET_VERSION=6.1
     - RACKET_VERSION=6.1.1
     - RACKET_VERSION=HEAD
+    - RACKET_VERSION=SCOPE_SNAPSHOT
+    
+matrix:
+  allow_failures:
+    - env: RACKET_VERSION=HEAD
+    - env: RACKET_VERSION=SCOPE_SNAPSHOT
+  fast_finish: true
 
 before_install:
   - git clone https://github.com/greghendershott/travis-racket.git ../travis-racket