Fix flushInternal not succeeding

[leyanlo]

This was broken in #158. Calling this.flushInternal() within a setInterval() fails because this does is not bound properly like it is for this.flush(). The result is that logging is never flushed, and we see a continuous stream of error messages every 5 seconds:

2019-01-23T23:58:59.012Z - error: Cannot read property 'getAndClear' of undefined message=Cannot read property 'getAndClear' of undefined, line=null, col=null, stack=TypeError: Cannot read property 'getAndClear' of undefined
    at flushInternal (http://localhost:3000/shipper/_static/client-vendor.js:37174:32), message=Cannot read property 'getAndClear' of undefined, captureType=browser, captureSource=client, framework=fusion

Verified that running the plugin with these changes properly flushes, including flushing before termination.

[KevinGrandon]

Makes sense, thanks!

[KevinGrandon]

(We should see if we can write a test case to address this, maybe doesn't block landing though)

[leyanlo]

Added a test case for setInterval, and changed all UniversalEmitter methods to arrow functions for consistency

[KevinGrandon]

Hello, thanks for the pull request, and sorry about the delay in landing. It looks like someone from our team jumped on this and landed. (We should've landed from this pull request as a base).

Apologies about that. You have done some good work on the testing here, feel free to open up a new PR if you think the tests would be valuable to update.

[KevinGrandon]

Relevant PR: #177

[leyanlo]

Ah no worries, thanks for the update!