travis: fetch trusty dependencies from bintray

The trusty dependencies are now hosted on Bintray.

.travis.yml

@@ -22,8 +22,15 @@ env:
 
 addons:
   apt:
+    sources:
+     - sourceline: 'deb https://dl.bintray.com/libgit2/ci-dependencies trusty libgit2deps'
+       key_url: 'https://www.edwardthomson.com/keys/ethomson@libgit2.org'
     packages:
     - cmake
+    - curl
+    - libcurl3
+    - libcurl3-gnutls
+    - libcurl4-gnutls-dev
     - libssh2-1-dev
     - openssh-client
     - openssh-server