Rephrased security information

Rephrased security information

The permission caching bug has been present forever, is presumably
going to stay around for a while, and is of less concern if
allow_other is not used. Since allow_other is disabled by default, I
think we can safely make this warning less prominent and document the
problem when we describe allow_other.

Also, drop the travis build status. It's confusing when reading
README.md after extracting the tarball, and I am not sure who benefits
from the build status when it is shown on GitHub either.