Use non-wildcard dependencies

[keirlawson]

Trying to use docular 0.8 within a project with NPM 3 I wound up with an ancient version of fs-extra which didn't have some of the methods that docular expects. Docular should explicitly specify minimum versions of its dependencies.

[wingedfox]

+1, by default npm 3 fetches fs-extra 0.8, which is not suitable