Segment Emitter, remove unsafe Buffer method

[thegareth]

No Issue

the Segment_emitter used the deprecated New Buffer(data) constructor which caused warnings on newer Node runtimes, and is noise in CloudWatch Logs.

[DEP0005] DeprecationWarning: Buffer() is deprecated due to security and usability issues. Please use the Buffer.alloc(), Buffer.allocUnsafe(), or Buffer.from() methods instead.

One buffer is used, and this has been replaced by the newer Buffer.from(data) construct.

This solution will not work for node version prior to 6: it's a simple replacement rather than a more complicated backwards compatible solution Detailed here.

Given that AWS only supports 8.10 or 10.x, I submit this anyway, and you can take a decision is compatibility is of concern.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[haotianw465]

We do have a plan of releasing a new major version of the SDK. The proposed change set includes dropping support of Node4 and 6. There is an issue open for collecting feedback: #157. This change will go with the new major version. We don't have an ETA yet but will update progress on that issue.