Travis: Make scons cache branch-specific

(cherry picked from commit a99b7a5f886a0462c79c321dcccf6c589f9d140d)

.travis.yml

@@ -6,7 +6,7 @@ sudo: false
 
 env:
   global:
-    - SCONS_CACHE=$HOME/.scons_cache
+    - SCONS_CACHE=$HOME/.scons_cache/$TRAVIS_BRANCH
     - SCONS_CACHE_LIMIT=1024
     - OPTIONS="debug_symbols=no verbose=yes progress=no"
     - secure: "uch9QszCgsl1qVbuzY41P7S2hWL2IiNFV4SbAYRCdi0oJ9MIu+pVyrQdpf3+jG4rH6j4Rffl+sN17Zz4dIDDioFL1JwqyCqyCyswR8uACC0Rr8gr4Mi3+HIRbv+2s2P4cIQq41JM8FJe84k9jLEMGCGh69w+ibCWoWs74CokYVA="