Tolerate dub noise

I had an interesting bug where flycheck-dmd-dub-set-include-path froze emacs for a while on a new project, then threw an error and failed to work until being run a second time explicitly. Turns out "dub describe" was fetching stuff and though it returned the JSON data, there was a bunch of junk before it, such as:

"The following changes will be performed:\nFetch vibe-d >=0.7.17, userWide\nFetch libevent ~master, userWide\nFetch openssl ~master,"

I've truncated that string to protect the innocent. Anyway I figured it might be nice to skip everything up to the first opening brace before trying to parse the JSON.

Cheers