travis: move back to the minimal image

The most recent update replaced the minimal image with a large one for the
'generic' image. Switching back to 'minimal' should reduce dependencies and
maybe speed us up some.

It should also eliminiate the need for aa2e0f0.

.travis.yml

@@ -6,7 +6,7 @@ addons:
   hostname: bitcoin-tester
 
 os: linux
-language: generic
+language: minimal
 cache:
   directories:
   - depends/built