Update shadow plugin in build.gradle

Needed to build this with the latest Gradle release.

build.gradle

@@ -26,7 +26,7 @@ buildscript {
     }
 
     dependencies {
-        classpath 'com.github.jengelman.gradle.plugins:shadow:1.2.0'
+        classpath 'com.github.jengelman.gradle.plugins:shadow:1.2.3'
         classpath 'org.jfrog.buildinfo:build-info-extractor-gradle:3.0.1'
         classpath 'org.ajoberstar:gradle-git:0.12.0'
     }