Reverting express-hbs to 0.5.1

- there is some sort of bug with 0.5.2 when combined with recent changes which results in the following error:

/home/travis/build/TryGhost/Ghost/node_modules/express-hbs/lib/hbs.js:128
      if(count < self.partialsDir.length){
                                 ^
TypeError: Cannot read property 'length' of undefined