update the gradle plugin for bintray. The old plugin failed when we upgraded to gradle 6.5 (#404)

Author: thomaszurkan-optimizely

build.gradle

@@ -1,23 +1,11 @@
-buildscript {
-    repositories {
-        jcenter()
-        maven {
-            url "https://oss.sonatype.org/content/repositories/snapshots/"
-        }
-    }
-
-    dependencies {
-        classpath 'com.jfrog.bintray.gradle:gradle-bintray-plugin:1.6'
-    }
-}
-
 plugins {
     id 'com.github.kt3k.coveralls' version '2.8.2'
     id 'jacoco'
     id 'me.champeau.gradle.jmh' version '0.4.5'
     id 'nebula.optional-base' version '3.2.0'
     id 'com.github.hierynomus.license' version '0.15.0'
     id 'com.github.spotbugs' version "4.5.0"
+    id "com.jfrog.bintray" version "1.8.5"
 }
 
 allprojects {
@@ -65,12 +53,12 @@ subprojects {
     }
 
     task sourcesJar(type: Jar, dependsOn: classes) {
-        classifier = 'sources'
+        archiveClassifier.set('sources')
         from sourceSets.main.allSource
     }
 
     task javadocJar(type: Jar, dependsOn: javadoc) {
-        classifier = 'javadoc'
+        archiveClassifier.set('javadoc')
         from javadoc.destinationDir
     }