Fix import paths to use gonum.org/v1/gonum

[jasonzhuyx]

Since December, 2018, both "gonum/blas" and "gonum/matrix" have been deprecated and migrated to "gonum/gonum".

• f22b278 (2018-12-08) blas: add deprecation notice
• c518dec (2018-12-09) matrix: add deprecation notice

As "go.mod" (at https://github.com/gonum/gonum) declares

module gonum.org/v1/gonum

this PR includes the changes to use "gonum.org/v1/gonum" as import paths in

• knn/knn.go
• linear_models/linear_regression.go
• metrics/pairwise/chebyshev.go
• metrics/pairwise/cranberra.go
• metrics/pairwise/manhattan.go

[codecov-io]

Codecov Report

Merging #237 into master will not change coverage.
The diff coverage is 75%.

@@           Coverage Diff           @@
##           master     #237   +/-   ##
=======================================
  Coverage   70.78%   70.78%           
=======================================
  Files          68       68           
  Lines        5945     5945           
=======================================
  Hits         4208     4208           
  Misses       1392     1392           
  Partials      345      345

Impacted Files	Coverage Δ
linear_models/linear_regression.go	93.84% <ø> (ø)	⬆️
knn/knn.go	76.09% <0%> (ø)	⬆️
metrics/pairwise/manhattan.go	100% <100%> (ø)	⬆️
metrics/pairwise/cranberra.go	100% <100%> (ø)	⬆️
metrics/pairwise/chebyshev.go	100% <100%> (ø)	⬆️

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