Add GitHub token as insecure environment variable

This should enable building via PR from forks.

This token has read-only access to download the installer packages, but
should nonetheless be revoked and replaced with a wget download when the
repository goes public.

.travis.yml

@@ -7,6 +7,8 @@ matrix:
     - os: osx
       osx_image: xcode8
       language: generic
+env:
+  - GITHUB_TOKEN=a27573e27bea23fb05393ead69511b09e3b224be
 cache:
   directories:
     - $HOME/.dotnet