Removed the global flag from the regular expression.

This fixes https://github.com/jsantell/node-json-front-matter/issues/3.

When `parseString` is called consecutively on two strings, the first one being invalid while the second one is valid, the `regexp.exec` call returns a valid match on the first string and the subsequent call with the second string returns null. I believe this is because of the global flag in the regexp, removing that flag fixes the issue.