chore(travis): wrap npm install with travis_retry to mitigate timeout failure

https://docs.travis-ci.com/user/common-build-problems/#timeouts-installing-dependencies

Author: Fonger

.travis.yml

@@ -1,6 +1,8 @@
 language: node_js
 sudo: false
 node_js: [10, 9, 8, 7, 6, 5, 4]
+install:
+  - travis_retry npm install
 before_script:
   - wget http://fastdl.mongodb.org/linux/mongodb-linux-x86_64-3.6.6.tgz
   - tar -zxvf mongodb-linux-x86_64-3.6.6.tgz