Prepare to publish null safety release (#18)

CHANGELOG.md

diff --git a/CHANGELOG.md b/CHANGELOG.md
index a4c55a5..5cc86fd 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,4 +1,4 @@
-## 0.2.0-dev
+## 0.2.0
 
 * Migrate to null safety.
 * Fix a number of lints affecting package maintenance score.

pubspec.yaml

diff --git a/pubspec.yaml b/pubspec.yaml
index 9988670..908463e 100644
--- a/pubspec.yaml
+++ b/pubspec.yaml
@@ -1,5 +1,5 @@
 name: http_retry
-version: 0.2.0-dev
+version: 0.2.0
 
 description: >-
   A wrapper for package:http clients that automatically retries requests