support android api level 21-

Author: BakerJQ

rxretrohttp/build.gradle

@@ -23,7 +23,7 @@ dependencies {
     api 'com.squareup.okhttp3:okhttp-urlconnection:3.11.0'
     api 'io.reactivex.rxjava2:rxjava:2.2.9'
     api 'io.reactivex.rxjava2:rxandroid:2.1.1'
-    api 'com.alibaba:fastjson:1.1.71.android'
+    api 'com.alibaba:fastjson:1.2.54'
     api 'com.squareup.retrofit2:adapter-rxjava2:2.8.1'
     api 'com.trello.rxlifecycle3:rxlifecycle:3.0.0'
     api 'com.trello.rxlifecycle3:rxlifecycle-android:3.0.0'