update rtdev:check_node/1 to work w/ atom version specifiers

previous specifiers were strings, changing to atoms (e.g. previous, legacy)
cause an error when generating the error message. Also, update message to
suggest that configuration could also be the problem.
Merge pull request #174 from basho/jrw-check-node-fix

update rtdev:check_node/1 to work w/ atom version specifiers