Fall back to empty object if project has no dependencies

Some projects have no dependencies, but only devDependencies. In these projects, brunch fails because of 
```
/var/www/jimdo/prod/node_modules/brunch/lib/watch.js:403
    plugins = loadDeps(Object.keys(json.dependencies));
                              ^
TypeError: Object.keys called on non-object
  at Function.keys (native)
  at loadPackages (/var/www/jimdo/prod/node_modules/brunch/lib/watch.js:403:31)
  at /var/www/jimdo/prod/node_modules/brunch/lib/watch.js:422:19
  at /var/www/jimdo/prod/node_modules/brunch/lib/helpers.js:498:16
  at /var/www/jimdo/prod/node_modules/brunch/lib/helpers.js:457:14
  at /var/www/jimdo/prod/node_modules/brunch/node_modules/read-components/index.js:240:16
  at /var/www/jimdo/prod/node_modules/brunch/node_modules/read-components/index.js:54:14
  at Object.cb [as oncomplete] (fs.js:168:19)
```
Merge pull request #823 from joscha/fix-empty-dependencies

Fall back to empty object if project has no dependencies