Include Sonatype snapshots repo only for snapshot builds

This prevents release builds from accidentally including snapshot
dependencies.

build.gradle

@@ -11,6 +11,8 @@ plugins {
 
 version = "1.0.1-SNAPSHOT"
 
+final isSnapshot = project.version.endsWith("-SNAPSHOT")
+
 final SOURCE_ENCODING = "UTF-8"
 
 wrapper {
@@ -20,11 +22,13 @@ wrapper {
 
 repositories {
 	jcenter()
+	if (isSnapshot) {
 		maven {
 			name("Sonatype snapshots")
 			url("https://oss.sonatype.org/content/repositories/snapshots")
 		}
 	}
+}
 
 configurations.all {
 	resolutionStrategy {