Don't run npm install in .travis.yml

This will prevent native dependencies from being built against the wrong
headers. `script/cibuild` will take care of installing all the package
dependencies using `apm`.

.travis.yml

@@ -13,6 +13,8 @@ notifications:
     rooms:
     - secure: 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
 
+install: true
+
 before_script:
   - rm -rf /usr/local/var/postgres
   - initdb /usr/local/var/postgres