Travis: Work around frequent build failures The build will consistently fail for days at a time with: error: http://pool.sks-keyservers.net/pks/lookup?op=get&search=0x0575F26BD5B3FDB1: import read failed(-1). I have a hunch that this is related to the CentOS 5 Docker container, so this commit causes Travis to download the RPM signing key outside of the container and share it with the container.