Dependency fast-json-stringify pulls deprecated dependency string-similarity

[tkalmar]

When installing the latest version of pino-gelf npm shows a deprecation warning for the string-similarity package.
This package is pulled in via fast-json-stringify package. This dependency is removed in version 5 or later.
Please update dependency to get rid of the deprecation warning.

[mcollina]

Thanks for reporting! Would you like to send a Pull Request to address this issue? Remember to add unit tests.

[tkalmar]

@mcollina What about #253 ??

[tkalmar]

@mcollina #260 this should fix the test errors introduced by the new version