ci(.travis.yml,appveyor.yml): Use current npm version in CI scripts (#…

…3579)

The requirement for using npm-5.x was due to npm-6 dropping support for Node-4. Since Mocha now requires Node-6+, we should use the current verion of npm. This will also eliminate npm
"Unsupported Node version" warnings when run against Node-11+.

.travis.yml

@@ -21,7 +21,7 @@ addons:
 before_install: |
   [[ ! -x ~/npm/node_modules/.bin/npm ]] && {
     # caching feature creates `~/npm` for us
-    cd ~/npm && npm install npm@^5
+    cd ~/npm && npm install npm
     cd -
   } || true
   # avoids bugs around https://github.com/travis-ci/travis-ci/issues/5092

appveyor.yml

@@ -37,7 +37,7 @@ install:
   - set PATH=%APPDATA%\npm;C:\MinGW\bin;%PATH%
   - ps: Install-Product node $env:nodejs_version x64
   - ps: Add-AppveyorMessage "Installing npm..."
-  - npm install -g npm@^5
+  - npm install -g npm
   ## Mocha-related package installs
   - ps: Add-AppveyorMessage "Installing Mocha dependencies..."
   - npm ci --ignore-scripts