prepare 2.15.0 release

[LaunchDarklyCI]

[2.15.0] - 2019-11-06

Changed:

• Changed the behavior of the warning message that is logged on failing to establish a streaming connection. Rather than the current behavior where the warning message appears upon each failed attempt, it will now only appear on the first failure in each series of attempts. Also, the message has been changed to mention that retries will occur. (#182)

Fixed:

• The beforeunload event handler no longer calls close on the client, which was causing the SDK to become unusable if the page did not actually close after this event fired (for instance if the browser navigated to a URL that launched an external application, or if another beforeunload handler cancelled leaving the page). Instead, it now only flushes events. There is also an unload handler that flushes any additional events that might have been created by any code that ran during the beforeunload stage. (#181)
• Removed uses of Object.assign that caused errors in Internet Explorer unless a polyfill for that function was present. These were removed earlier in the 2.1.1 release, but had been mistakenly added again.
• Removed development dependency on typedoc which caused some vulnerability warnings.

Deprecated:

• The samplingInterval configuration property is deprecated and will be removed in a future version. The intended use case for the samplingInterval feature was to reduce analytics event network usage in high-traffic applications. This feature is being deprecated in favor of summary counters, which are meant to track all events.