travis: we use bintray's own key for signing

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

(cherry picked from commit 6e748130)

.travis.yml

@@ -28,7 +28,7 @@ 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