fix (proguard): added a progruard keep rule for slf4j (#312)

* added a progruard keep rule for slf4j

* prepare for release. i will cherry-pick this one commit to master as well.

Author: thomaszurkan-optimizely

CHANGELOG.md

@@ -1,5 +1,11 @@
 # Optimizely Android X SDK Changelog
 
+## 3.1.2
+January 7th, 2020
+
+### Bug Fixes:
+* Add keep for log4j to proguard rules.
+
 ## 3.1.1
 July 23rd, 2019
 

proguard-rules.txt

@@ -55,6 +55,7 @@
 
 # slf4j
 -dontwarn org.slf4j.**
+-keep class org.slf4j.** {*;}
 
 # Android Logger
 -keep class com.noveogroup.android.log.** { *; }