@obfuscurity => Check for undefined 'usingCloudWatch' value #minor

Sorry about this followup to https://github.com/obfuscurity/tasseo/pull/91. Forgot to check with an example that did not use Cloudwatch.

This fix will prevent a `Uncaught ReferenceError: usingCloudWatch is not defined` error, and thus not require non-Cloudwatch sources to set `var usingCloudWatch = false;` in their metric configurations.

Sorry for the oversight!