Uninstall encoding before first Travis test run

encoding can be included in the Travis CI cache, and thus needs to be
manually uninstalled first.

Author: TimothyGu

.travis.yml

@@ -10,6 +10,7 @@ env:
 before_script:
   - 'if [ "$FORMDATA_VERSION" ]; then npm install form-data@^$FORMDATA_VERSION; fi'
 script:
+  - npm uninstall encoding
   - npm run coverage
   - npm install encoding
   - npm run coverage