Fix math_test.go for Go1.9

[dsnet]

https://golang.org/cl/39152 fixes the testing/quick package to properly generate
all possible uint64 values which affects the range of values that are generated
for float32. This causes some inputs to be provided that exceeds the 1e-7
tolerance previously set. Instead, we increase it to 1e-6.

[dsnet]

\cc @kortschak

[kortschak]

Thanks @dsnet. Most of the old gonum packages have been moved over to gonum.org/v1/gonum and are now housed together in github.com/gonum/gonum, so we have stopped development on these repos (we need to put up a CONTRIBUTING.md to let people know they are deprecated and note in the README.md - our focus has been elsewhere).

A change similar to this, targeting only the Hypot function has been merged into gonum.org/v1/gonum.