Try more keyservers to fix unstable Travis builds

[lwille]

Travis jobs to build the CI images are failing regularly because ha.pool.sks-keyservers.net seems unavailable (https://travis-ci.org/docker-library/wordpress/jobs/338804185):

+ gpg --keyserver ha.pool.sks-keyservers.net --recv-keys 3B9191625F3B1F1BF5DD3B47673A02042F6B6B7F
gpg: keybox '/tmp/tmp.IcLNCf/pubring.kbx' created
gpg: keyserver receive failed: Address not available

I modified the Dockerfiles to try three different pools in an attempt to mitigate the problem.

[yosifkit]

My current opinion of this is expressed in docker-library/cassandra#131 (comment).

Related issues: docker-library/tomcat#87, docker-library/tomcat#108, docker-library/mysql#263 (comment), docker-library/httpd#66 (comment), docker-library/php#586