travis: Use the generic VM to test conda-smithy.

We don't actually care what VM we get from Travis CI or what comes with
it as we install Miniconda and use its Python and assorted packages for
constructing a testing environment. As a result, it would be better if
we use one of the "cheap" Travis VM images to start as we don't care
what comes with it. Here we propose using the `generic` image, which we
have used successfully in a lot of other cases. Also it has very fast
boot times. This should be helpful for iterating quickly on proposed
changes.

.travis.yml

@@ -1,6 +1,6 @@
 # The language in this case has no bearing - we are going to be making use of conda for a
 # python distribution for the scientific python stack.
-language: python
+language: generic
 
 sudo: false