revert to using npm 4

Author: kiwiupover

.travis.yml

@@ -3,7 +3,6 @@ language: node_js
 node_js:
   - "6"
   - "8"
-  - "10"
 
 branches:
   only:
@@ -31,6 +30,5 @@ cache:
   yarn: false # switch to yarn once ember-cli-addon-tests is removed
 
 before_install:
-
-  # TODO: remove when we stop testing against node 6
-  - if [[ `npm -v` < 6* ]]; then npm i -g npm@6; fi
+  - npm config set spin false
+  - npm install -g npm@4

appveyor.yml

@@ -6,7 +6,6 @@ environment:
   matrix:
     - nodejs_version: "6"
     - nodejs_version: "8"
-    - nodejs_version: "10"
 
 branches:
   only:
@@ -17,7 +16,8 @@ branches:
 # Install scripts. (runs after repo cloning)
 install:
   - ps: Install-Product node $env:nodejs_version
-  - appveyor-retry npm install -g npm@^6
+  - npm config set spin false
+  - appveyor-retry npm install -g npm@^3
   - appveyor-retry npm i -g bower # for ember-cli-addon-tests
   - npm --version
   - appveyor-retry choco install phantomjs