Add generics to randomGenerator.element()

[bobjackman]

This change allows code like this, that would normally cause either cannot find declaration to go to or getter `values` isn't defined for the class Object, to resolve properly:

var items = [
  {'a': 'foo', 'b': 'bar'},
  {'a': 'zip', 'b': 'zap'}
]
var chosenItem = faker.randomGenerator.element(items).values;

[bobjackman]

@drager it looks like the failed check above is due to a bad configuration for TravisCI...

$ ./tool/travis.sh
Setting VM flags failed: Unrecognized flags: checked

https://travis-ci.org/drager/faker/jobs/528987224

[drager]

Thanks for the PR @kogi! I think the code looks good! I will check out the Travis error.