Upgrade Error Prone fork v2.20.0-picnic-1 -> v2.20.0-picnic-2 (#725)

This new release is a workaround for jitpack/jitpack.io#5741.

See PicnicSupermarket/error-prone@v2.20.0-picnic-1...v2.20.0-picnic-2

pom.xml

@@ -200,7 +200,7 @@
         <version.auto-service>1.1.1</version.auto-service>
         <version.auto-value>1.10.2</version.auto-value>
         <version.error-prone>${version.error-prone-orig}</version.error-prone>
-        <version.error-prone-fork>v${version.error-prone-orig}-picnic-1</version.error-prone-fork>
+        <version.error-prone-fork>v${version.error-prone-orig}-picnic-2</version.error-prone-fork>
         <version.error-prone-orig>2.20.0</version.error-prone-orig>
         <version.error-prone-slf4j>0.1.18</version.error-prone-slf4j>
         <version.guava-beta-checker>1.0</version.guava-beta-checker>