Update npm before building on AppVeyor/Travis

.travis.yml

@@ -23,6 +23,7 @@ install:
   - source /tmp/.nvm/nvm.sh
   - nvm install $NODE_VERSION
   - nvm use --delete-prefix $NODE_VERSION
+  - npm install -g npm
   - script/bootstrap
   - script/build
 

appveyor.yml

@@ -20,6 +20,7 @@ environment:
 install:
   - SET PATH=C:\Program Files\Atom\resources\cli;%PATH%
   - ps: Install-Product node $env:NODE_VERSION $env:PLATFORM
+  - npm install -g npm
 
 build_script:
   - cd %APPVEYOR_BUILD_FOLDER%