Run tests against different ESLint versions on Travis

Author: yannickcr

.travis.yml

@@ -9,6 +9,8 @@ node_js:
   - '4'
 before_install:
   - 'nvm install-latest-npm'
+before_script:
+  - 'if [ -n "${ESLINT-}" ]; then npm install --no-save "eslint@${ESLINT}" ; fi'
 script:
   - 'if [ -n "${PRETEST-}" ]; then npm run pretest ; fi'
   - 'if [ -n "${TEST-}" ]; then npm run unit-test ; fi'
@@ -21,7 +23,28 @@ matrix:
   include:
     - node_js: 'lts/*'
       env: PRETEST=true
+    - node_js: '9'
+      env: TEST=true ESLINT=next
+    - node_js: '8'
+      env: TEST=true ESLINT=next
+    - node_js: '7'
+      env: TEST=true ESLINT=next
+    - node_js: '6'
+      env: TEST=true ESLINT=next
+    - node_js: '9'
+      env: TEST=true ESLINT=3
+    - node_js: '8'
+      env: TEST=true ESLINT=3
+    - node_js: '7'
+      env: TEST=true ESLINT=3
+    - node_js: '6'
+      env: TEST=true ESLINT=3
+    - node_js: '5'
+      env: TEST=true ESLINT=3
+    - node_js: '4'
+      env: TEST=true ESLINT=3
   allow_failures:
     - node_js: '9'
     - node_js: '7'
     - node_js: '5'
+    - env: TEST=true ESLINT=next