Do not JSON-decode tag rules twice

The Navigate::extract_command method calls JSON::parse twice when passed
the create-tag command's "rule" argument value, which triggers an exception
and crashes the client. The second call is unnecessary, so this patch
removes it.
* README.md: add pointer to JIRA bug tracker
* razor-client.gemspec: require at least Ruby 1.9.3
* bin/razor: set exit code on error; print nice error for invalid URL

  * Explicitly exit with error code 1 if there are any errors
  * Do not print a backtrace when API URL is invalid