Travis CI fix (#366)

* a quick test to see if we can drop npm install before script
* adding node 8 lts

.travis.yml

@@ -2,6 +2,7 @@ language: node_js
 node_js:
   - "4"
   - "6"
+  - "8"
   - "node"
 env:
   - FORMDATA_VERSION=1.0.0
@@ -11,7 +12,6 @@ env:
 before_script:
   - 'if [ "$FORMDATA_VERSION" ]; then npm install form-data@^$FORMDATA_VERSION; fi'
   - 'if [ "$ENCODING" = "yes" ]; then npm install encoding; fi'
-before_install: npm install -g npm
 script: npm run coverage
 cache:
   directories: