fix: package.json to reduce vulnerabilities
fix: package.json to reduce vulnerabilities

The following vulnerabilities are fixed with an upgrade:
- https://snyk.io/vuln/npm:marked:20170815
- https://snyk.io/vuln/npm:marked:20170815-1

Latest report for streetsupport/manchester-charter:
https://snyk.io/test/github/streetsupport/manchester-charter

Some vulnerabilities weren't fixed or ignored, and so will still fail
the Snyk test report.
StreetSupport/manchester-charter,StreetSupport/manchester-charter,StreetSupport/manchester-charter