Remove istanbul from package.json

[shamrickus]

Since Istanbul is no longer supported, this is producing npm audit warnings

npm WARN deprecated istanbul@0.4.5: This module is no longer maintained, try this instead:
npm WARN deprecated   npm i nyc
npm WARN deprecated Visit https://istanbul.js.org/integrations for other alternatives.

This PR removes it form package.json. Since travis already installs the package manually this should not break CI, and all tests still pass

It also updates package-lock.json as it appears that was not updated with the 0.8 release

[coveralls]

Coverage remained the same at 94.372% when pulling 220c7a6 on shamrickus:remove-istanbul into 38995eb on rquadling:master.

[rquadling]

Thank you.