Travis: Avoid apt complaining about public key 6B05F25D762E3157

Also see

git-lfs/git-lfs#3474 (comment)

.travis.yml

@@ -158,6 +158,7 @@ before_install:
   - eval "${MATRIX_EVAL}"
   - sudo rm -vf /etc/apt/sources.list.d/*rabbit*
   - sudo add-apt-repository -y ppa:ubuntugis/ubuntugis-unstable
+  - sudo apt-key adv --keyserver keyserver.ubuntu.com --recv-keys 6B05F25D762E3157
   - sudo apt update
   - sudo apt install libgdal-dev gdal-bin