Publish release builds to Maven Central automatically

Adds the nexus-staging-plugin which allows to automatically close and
release the staging repository on OSSRH.

Author: cboehme

.travis.yml

@@ -20,6 +20,7 @@ script:
 
 after_success:
   - ./travis/publish.sh
+  - ./gradlew closeAndReleaseRepository
 
 deploy:
   provider: releases

build.gradle

@@ -19,6 +19,7 @@ import java.util.regex.Matcher
 plugins {
   id 'org.ajoberstar.grgit' version '2.0.1'
   id 'org.sonarqube' version '2.5'
+  id 'io.codearte.nexus-staging' version '0.11.0'
 }
 
 ext.scmInfo = getScmInfo()
@@ -182,6 +183,24 @@ sonarqube {
   }
 }
 
+nexusStaging {
+  if (project.hasProperty('stagingServerUrl')) {
+    serverUrl = stagingServerUrl
+  }
+  if (project.hasProperty('releaseRepositoryUser')) {
+    username = releaseRepositoryUser
+    password = releaseRepositoryPassword
+  }
+}
+
+tasks.closeRepository.onlyIf {
+  rootProject.scmInfo.isRelease()
+}
+
+tasks.releaseRepository.onlyIf {
+  rootProject.scmInfo.isRelease()
+}
+
 class ScmInfo {
   def version
   def tag