ci: update npm to 5.8.0

Changes in project dependency structure were causing `npm` 5.6.0 to incorrectly flag `fsevents` as a required dependency of `chokidar` 2.0.3 and 1.7.0, but it is actually an optional dependency.

This doesn't seem to happen with `npm` 5.8.0.

Author: filipesilva

.appveyor.yml

@@ -7,6 +7,7 @@ matrix:
 
 install:
   - ps: Install-Product node $env:nodejs_version
+  - npm install -g npm@5.8.0
   - npm install
   - choco install googlechrome
 

.travis.yml

@@ -93,6 +93,7 @@ matrix:
 
 
 install:
+  - npm install -g npm@5.8.0
   - npm install
 
 env: