Revert "Merge pull request #317 from prometheus/fix/miekg-dns-for-srv"

This reverts the introduction of our own miekg/dns based resolver.
Since the missing tcp retry on long responses is fixed (https://code.google.com/p/go/issues/detail?id=5686), we can drop this external dependency.
Beside that, LookupSRV respects the local domain search path which our resolver based on miekg/dns does not.