resolve rubocop offense in latest version

Rubocop says:

> Pass &:chomp as an argument to map! instead of a block.
resolve CVE-2014-6271 and CVE-2014-7169 (shellshock)

* http://web.nvd.nist.gov/view/vuln/detail?vulnId=CVE-2014-6271
* http://web.nvd.nist.gov/view/vuln/detail?vulnId=CVE-2014-7169
* https://www.duosecurity.com/blog/critical-vulnerability-discovered-in-bash

resolves https://github.com/jumanjiman/wormhole/issues/38