Consolidate Travis CI matrix for encoding (node-fetch#436)

See: node-fetch#431

Author: TimothyGu

.travis.yml

@@ -6,13 +6,13 @@ node_js:
   - "node"
 env:
   - FORMDATA_VERSION=1.0.0
-  - FORMDATA_VERSION=1.0.0 ENCODING=yes
   - FORMDATA_VERSION=2.1.0
-  - FORMDATA_VERSION=2.1.0 ENCODING=yes
 before_script:
   - 'if [ "$FORMDATA_VERSION" ]; then npm install form-data@^$FORMDATA_VERSION; fi'
-  - 'if [ "$ENCODING" = "yes" ]; then npm install encoding; fi'
-script: npm run coverage
+script:
+  - npm run coverage
+  - npm install encoding
+  - npm run coverage
 cache:
   directories:
     - node_modules