Downgrade okhttp to 3.12.6

https://github.com/square/okhttp/issues/5713
https://stackoverflow.com/questions/8183205/what-could-be-the-cause-of-rejectedexecutionexception/8183463

build.gradle

@@ -10,7 +10,7 @@ ext {
     GIT_URL = 'https://github.com/wulkanowy/sdk.git'
 
     jspoon = "1.3.2"
-    okhttp3 = "3.12.7"
+    okhttp3 = "3.12.6"
     retrofit = "2.6.4"
     threetenbp = "1.4.0"
     slf4j = "1.7.30"
@@ -128,4 +128,6 @@ dependencies {
     compile project(":sdk")
     compile project(":sdk-mobile")
     compile project(":sdk-scrapper")
+
+    compile "com.squareup.okhttp3:okhttp:$okhttp3"
 }