travis: we use bintray's own key for signing

The VM on Travis apparently will still proceed, but it's good practice.

.travis.yml

@@ -24,7 +24,7 @@ addons:
   apt:
     sources:
      - sourceline: 'deb https://dl.bintray.com/libgit2/ci-dependencies trusty libgit2deps'
-       key_url: 'https://www.edwardthomson.com/keys/ethomson@libgit2.org'
+       key_url: 'https://bintray.com/user/downloadSubjectPublicKey?username=bintray'
     packages:
     - cmake
     - curl