Code cleaning & avoid stack overflow for recursive serialization

[thefirstspine]

1/ The clone function is not used anywhere in the dependency - we deleted it

2/ Serialization is throwing a stack overflow exception for recursive objects:

RangeError: Maximum call stack size exceeded
    at Function.valueOf (<anonymous>)
    at Object.common.serialize (/<app>/node_modules/node-loggly-bulk/lib/loggly/common.js:399:28)
    at Object.common.serialize (/<app>/node_modules/node-loggly-bulk/lib/loggly/common.js:420:21)
    at Object.common.serialize (/<app>/node_modules/node-loggly-bulk/lib/loggly/common.js:420:21)
    at Object.common.serialize (/<app>/node_modules/node-loggly-bulk/lib/loggly/common.js:420:21)
    at Object.common.serialize (/<app>/node_modules/node-loggly-bulk/lib/loggly/common.js:420:21)
    at Object.common.serialize (/<app>/node_modules/node-loggly-bulk/lib/loggly/common.js:420:21)
    at Object.common.serialize (/<app>/node_modules/node-loggly-bulk/lib/loggly/common.js:420:21)
    at Object.common.serialize (/<app>/node_modules/node-loggly-bulk/lib/loggly/common.js:420:21)
    at Object.common.serialize (/<app>/node_modules/node-loggly-bulk/lib/loggly/common.js:420:21)

We added a max depth for avoiding it

---

Tests are passing

 PASS  test/logs.test.js
  when trying to send logs
    ✓ should set correct token, URL, tags, headers and response (4 ms)
    ✓ should work with a non-json message
    ✓ should work with a json message
    ✓ should retry several times when error occurs (11 ms)

Test Suites: 1 passed, 1 total
Tests:       4 passed, 4 total
Snapshots:   0 total
Time:        0.797 s
Ran all test suites.