fit: optimize.Local removed from gonum

Created by: SpencerPark

See https://github.com/gonum/gonum/pull/538

Getting go/src/go-hep.org/x/hep/fit/curve1d.go:28:9: undefined: optimize.Local when running go get -insecure go-hep.org/x/hep/fit (in the gophernotes Dockerfile.DS).