Use BOLT_PROJECT_ROOT_DIR constant in setTwigPath

We're using bolt based off https://github.com/richardhinkamp/bolt-skeleton, and having the twig path built the way it is results in twig files being served from the vendor directory, rather than the project root. Using the constant alleviates the need to edit themes in the vendor directory.
Merge pull request #614 from sorabji52/patch-1

Use BOLT_PROJECT_ROOT_DIR constant in setTwigPath