Relax protobuf version requirement

[david4096]

Allows us to take advantage of newer versions of the protobuf module without modifying the requirements. If a specific version of the module is required at release, this can be set to a known working value.

Now that we have automated nightly CI (https://docs.travis-ci.com/user/cron-jobs/), this will make it easier manage changes to our dependencies.

Goes with https://github.com/ga4gh/server/pull/1557