Tolerate dub noise remix

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.

Aren't elisp regexps cute.

Cheers
It would be better to do this in `fldd--get-dub-package-dirs-json` and add a corresponding unit test in the `tests` directory. Another test case in `test-fldd--get-dub-package-dirs-json` would do.

The more is unit testable the better. Thanks.