Try to fix travis failure on doxygen stage

The Travis log included the following lines:

W: GPG error: http://packages.cloud.google.com/apt cloud-sdk-trusty InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 6A030B21BA07F4FB
W: The repository 'http://packages.cloud.google.com/apt cloud-sdk-trusty InRelease' is not signed.
W: There is no public key available for the following key IDs:
6A030B21BA07F4FB
W: http://ppa.launchpad.net/couchdb/stable/ubuntu/dists/trusty/Release.gpg: Signature by key 15866BAFD9BCC4F3C1E0DFC7D69548E1C17EAB57 uses weak digest algorithm (SHA1)
travis_fold:end:apt-get.diagnostics
�[0K
�[31;1mThe command "sudo -E apt-get -yq --no-install-suggests --no-install-recommends $TRAVIS_APT_OPTS install cmake google-cloud-sdk graphviz" failed and exited with 100 during .�[0m

Author: Owen

.travis.yml

@@ -49,18 +49,14 @@ jobs:
       env:
         NAME: "DOXYGEN-CHECK"
         DOXYGEN_VERSION: "1.8.14"
-      addons:
-        apt:
-          sources:
-            - sourceline: 'deb http://packages.cloud.google.com/apt cloud-sdk-trusty main'
-              key_url: 'https://packages.cloud.google.com/apt/doc/apt-key.gpg'
-          packages:
-            - cmake
-            - google-cloud-sdk
-            - graphviz
       cache:
         directories:
           - ${TRAVIS_BUILD_DIR}/doxygen/build/bin
+      before_install:
+        - curl -sSL "https://packages.cloud.google.com/apt/doc/apt-key.gpg" | sudo -E apt-key add -
+        - echo "deb http://packages.cloud.google.com/apt cloud-sdk-trusty main" | sudo tee -a /etc/apt/sources.list > /dev/null
+        - sudo apt-get update --option Acquire::Retries=100 --option Acquire::http::Timeout="60"
+        - sudo -E apt-get -yq --no-install-suggests --no-install-recommends --force-yes install cmake google-cloud-sdk graphviz
       install:
         - |
           # Build doxygen if it is not in Travis cache