all: migrate import paths to gonum.org/v1/plot

as discussed elsewhere [ref needed], we want to migrate the import path of github.com/gonum/plot to gonum.org/v1/plot.

contrary to the migration of github.com/gonum/{matrix,stat,...} to gonum.org/v1/gonum/{mat,stat,...} this can't be done w/o breaking people importing github.com/gonum/plot.

the fix is easy though:

$> find $GOPATH/src/github.com/me/pkg \
  -type f \
  -exec sed -i -e 's|"github.com/gonum/plot|"gonum.org/v1/plot|g' {} \;