Update magefile to use Go Dep
Update magefile to use Go Dep

See #3988

gohugoio/hugo,mdhender/hugo,gohugoio/hugo,anthonyfok/hugo,fedelibre/hugo,gohugoio/hugo,mdhender/hugo,fedelibre/hugo,fedelibre/hugo,mdhender/hugo,mdhender/hugo,fedelibre/hugo,anthonyfok/hugo,anthonyfok/hugo