Update the CI configuration

* Use the bundled Node.js and stop forcing install of Node.js v6

.travis.yml

@@ -1,28 +1,22 @@
 ### Project specific config ###
-
-# Installed for linting the project
-language: node_js
+language: generic
 
 matrix:
   include:
     - os: linux
-      node_js: "6"
       env: ATOM_CHANNEL=stable
 
     - os: linux
-      node_js: "6"
       env: ATOM_CHANNEL=beta
 
     - os: osx
-      node_js: "6"
       env: ATOM_CHANNEL=stable
 
 env:
   global:
     # Pre-install the required language file as package activation doesn't wait
     # for the installation to complete.
     - APM_TEST_PACKAGES="language-postcss"
-    - ATOM_LINT_WITH_BUNDLED_NODE="false"
 
 ### Generic setup follows ###
 script: 'curl -s https://raw.githubusercontent.com/atom/ci/master/build-package.sh | sh'

appveyor.yml

@@ -1,16 +1,11 @@
 ### Project specific config ###
 environment:
   APM_TEST_PACKAGES: language-postcss
-  ATOM_LINT_WITH_BUNDLED_NODE: "false"
 
   matrix:
   - ATOM_CHANNEL: stable
   - ATOM_CHANNEL: beta
 
-install:
-  # Install Node.js to run any configured linters
-  - ps: Install-Product node 6
-
 ### Generic setup follows ###
 build_script:
   - ps: iex ((new-object net.webclient).DownloadString('https://raw.githubusercontent.com/atom/ci/master/build-package.ps1'))

circle.yml

@@ -4,13 +4,11 @@ test:
 
 dependencies:
   override:
+    # If nothing is put here, CircleCI will run `npm install` using the system Node.js
     - echo "Managed in the script"
 
 machine:
-  node:
-    version: 6
   environment:
-    ATOM_LINT_WITH_BUNDLED_NODE: "false"
     # Pre-install the required language file as package activation doesn't wait
     # for the installation to complete.
     APM_TEST_PACKAGES: "language-postcss"