fix deprecation warnings in Node 8

[vigneshshanmugam]

Node 8 provides deprecation warnings when running with --pending-deprecation flag and we need small fixes in tailor core.

• Move from Buffer() to Buffer.allocUnsafe (https://nodejs.org/en/blog/release/v8.0.0/#buffer-changes)
• Include Node 8 as part of travis build

[addityasingh]

@vigneshshanmugam IMO we should use Buffer.allocUnsafe() instead of Buffer.alloc() to gain some performance in memory allocation. Ideally, we can create some benchmark tests to compare these 2, but I think Tailor doesn't have scenarios where memory leak can happen because of Buffer.allocUnsafe()`. Thoughts?

[vigneshshanmugam]

Yup, allowUnsafe should be the right way. Updated the list.

Memory leak can happen in tailor due to number of subscribers to tailor's event emitters. This might be a hint.