Fix Changelog Dates (census-instrumentation#497)

Based on the file's history, I believe this was actually supposed to read `2019`.

Author: brennanpayne

CHANGELOG.md

@@ -3,14 +3,14 @@
 ## Unreleased
 
 ## 0.2.0
-Released 2018-01-18
+Released 2019-01-18
 
 - Fix multiple stackdriver and prometheus exporter bugs
 - Increase size of trace batches and change transport behavior on exit
   ([#452](https://github.com/census-instrumentation/opencensus-python/pull/452))
 
 ## 0.1.11
-Released 2018-01-16
+Released 2019-01-16
 
 - Fix a bug in the stackdriver exporter that caused spans to be exported
   individually