chore: prepare for release 3.8.1 (#284)

## Summary
prepare for release 3.8.1

## Test plan
All Full stack compatibility tests and unit tests pass.

Author: ozayr-zaviar

CHANGELOG.md

@@ -1,5 +1,11 @@
 # Optimizely Ruby SDK Changelog
 
+## 3.8.1
+August 2nd, 2021
+
+### Bug Fixes:
+- Fixed duplicate experiment key issue with multiple feature flags. While trying to get variation from the variationKeyMap, it was unable to find because the latest experimentKey was overriding the previous one. [#282](https://github.com/optimizely/ruby-sdk/pull/282)
+
 ## 3.8.0
 February 16th, 2021
 

lib/optimizely/version.rb

@@ -17,5 +17,5 @@
 #
 module Optimizely
   CLIENT_ENGINE = 'ruby-sdk'
-  VERSION = '3.8.0'
+  VERSION = '3.8.1'
 end

spec/project_spec.rb

@@ -3625,7 +3625,7 @@ def callback(_args); end
           project_id: '111001',
           revision: '42',
           client_name: 'ruby-sdk',
-          client_version: '3.8.0',
+          client_version: '3.8.1',
           anonymize_ip: false,
           enrich_decisions: true,
           visitors: [{
@@ -3771,7 +3771,7 @@ def callback(_args); end
           project_id: '111001',
           revision: '42',
           client_name: 'ruby-sdk',
-          client_version: '3.8.0',
+          client_version: '3.8.1',
           anonymize_ip: false,
           enrich_decisions: true,
           visitors: [{