Bump version to 2.6.4

Author: zjn0505

xkcd/build.gradle

@@ -54,8 +54,8 @@ android {
         applicationId "xyz.jienan.xkcd"
         minSdkVersion 16
         targetSdkVersion 29
-        versionCode 43
-        versionName "2.6.3"
+        versionCode 44
+        versionName "2.6.4"
         vectorDrawables.useSupportLibrary = true
         resConfigs "zh-rCN"
         testInstrumentationRunner "androidx.test.runner.AndroidJUnitRunner"