Add a scopeless access token to public Travis config.

It may seem strange to commit an access token to a public repository but this
one has been granted no scopes, so it doesn't have access to *do* anything, and
is simply used to avoid the GH API's unauthed rate limiting when fetching public
assets. This path was chosen on the advice of the GH platform team.

.travis.yml

@@ -3,6 +3,7 @@ git:
 
 env:
   - NODE_VERSION=0.12
+  - ATOM_ACCESS_TOKEN=da809a6077bb1b0aa7c5623f7b2d5f1fec2faae4
 
 os:
   - linux