Excavator: Update open-source publishing plugins (#386)

build.gradle

@@ -5,7 +5,7 @@ buildscript {
     }
 
     dependencies {
-        classpath 'com.palantir.gradle.externalpublish:gradle-external-publish-plugin:1.14.0'
+        classpath 'com.palantir.gradle.externalpublish:gradle-external-publish-plugin:1.15.0'
         classpath 'com.palantir.gradle.gitversion:gradle-git-version:3.0.0'
     }
 }