[Tests] test on both eslint v3 and v4

Author: ljharb

.travis.yml

@@ -11,6 +11,8 @@ cache:
   yarn: true
   directories:
     - node_modules
+before_script:
+  - if [ -n "${ESLINT-}" ]; then npm install --no-save "eslint@${ESLINT}"; fi
 script:
   - if [ "${FLOW-}" = true ]; then npm run flow; fi
   - if [ "${LINT-}" = true ]; then npm run lint; fi
@@ -21,6 +23,9 @@ sudo: false
 env:
   global:
     - TEST=true
+  matrix:
+    - ESLINT=4
+    - ESLINT=3
 matrix:
   fast_finish: true
   include:

package.json

@@ -40,7 +40,7 @@
     "babel-polyfill": "^6.26.0",
     "babel-preset-es2015": "^6.24.1",
     "coveralls": "^3.0.1",
-    "eslint": "^4.19.1",
+    "eslint": "^3 || ^4",
     "eslint-config-airbnb-base": "^13.0.0",
     "eslint-plugin-flowtype": "^2.49.3",
     "eslint-plugin-import": "^2.13.0",