encrypt: Don't print the warning message if argument includes "="

If the environment variable includes "/", the warning message about the
-r option is printed. As far as I know, "=" is not a legal character in
the reposiory slug (it'll be changed to -), so we can avoid printing the
message if the argument included a "=").
Merge pull request #68 from travis-ci/hh-smarter-encrypt-warning-message

encrypt: Don't print the warning message if argument includes "="