Remove load-grunt-tasks dependency and requirement

load-grunt-config requires by default load-grunt-tasks, this double require
leads to a "TypeError: Object #<Object> has no method 'init'" when trying
to require s3 task for the second time.

See also https://github.com/pifantastic/grunt-s3/issues/68
Merge pull request #212 from potomak/fix-s3-task-error-has-no-method-init

Remove load-grunt-tasks dependency and requirement