Handle EOFError from socket closing unexpectedly.

Without this, an unexpected socket close results in:

CheckBanner CRITICAL: Check failed to run: end of file reached, ["/etc/sensu/plugins/network/check-banner.rb:56:in `readline'", "/etc/sensu/plugins/network/check-banner.rb:56:in `block in get_banner'", "/opt/sensu/embedded/lib/ruby/2.0.0/timeout.rb:66:in `timeout'", "/opt/sensu/embedded/lib/ruby/2.0.0/timeout.rb:97:in `timeout'", "/etc/sensu/plugins/network/check-banner.rb:53:in `get_banner'", "/etc/sensu/plugins/network/check-banner.rb:68:in `run'", "/opt/sensu/embedded/lib/ruby/gems/2.0.0/gems/sensu-plugin-0.2.1/lib/sensu-plugin/cli.rb:56:in `block in <class:CLI>'"]