Fix potential security vulnerabilities

[enriquetuya]

Fix the potential vulnerabilities displayed here: https://github.com/GlobalFishingWatch/map-client/network/dependencies

[nerik]

This is what npm ls shows for the problematic dependency:

└─┬ node-sass@4.9.2
  └─┬ node-gyp@3.7.0
    └─┬ request@2.81.0
      └─┬ hawk@3.1.3
        ├─┬ boom@2.10.1
        │ └── hoek@2.16.3  deduped
        ├── hoek@2.16.3
        └─┬ sntp@1.0.9
          └── hoek@2.16.3  deduped

hoek is a dependency of node-gyp, which is a dependency of node-sass. We need to wait for this PR to be merged: nodejs/node-gyp#1492, and then for the node-sass people to bump their node-gyp version.

[nerik]

sass/node-sass#2472
d21c71e