在IDEA导入Gradle项目之后一直报错

代码用的Spring Boot+ MyBatis
导入后一直报如下错误

Could not find org.springframework.boot:spring-boot-gradle-plugin:1.5.7.RELEASE.
Searched in the following locations:
    http://maven.aliyun.com/nexus/content/groups/public/org/springframework/boot/spring-boot-gradle-plugin/1.5.7.RELEASE/spring-boot-gradle-plugin-1.5.7.RELEASE.pom
    http://maven.aliyun.com/nexus/content/groups/public/org/springframework/boot/spring-boot-gradle-plugin/1.5.7.RELEASE/spring-boot-gradle-plugin-1.5.7.RELEASE.jar
Required by:
    project :

Gradle相关配置如下

apply plugin: 'java'
apply plugin: 'eclipse-wtp'
apply plugin: 'org.springframework.boot'
if (!DEVELOP_MODE.toBoolean()) {
    apply plugin: 'war'
}

group = 'com.server'
version = '0.0.1-SNAPSHOT'
sourceCompatibility = 1.8

repositories {
    maven { url 'http://maven.aliyun.com/nexus/content/groups/public/' }
}

configurations {
    providedRuntime
}

dependencies {
    compile('org.springframework.boot:spring-boot-starter-aop')
    compile('org.springframework.boot:spring-boot-starter-data-jpa')
    compile('org.springframework.boot:spring-boot-starter-jdbc')
    compile('org.springframework.boot:spring-boot-starter-web')
    compile('org.mybatis.spring.boot:mybatis-spring-boot-starter:1.3.1')
    compile "org.springframework.boot:spring-boot-starter-thymeleaf"
    runtime('mysql:mysql-connector-java')
    if (!DEVELOP_MODE.toBoolean()) {
        providedRuntime('org.springframework.boot:spring-boot-starter-tomcat')
    }
    testCompile('org.springframework.boot:spring-boot-starter-test')
}

https://www.jianshu.com/p/f1fb53a494ab

1，spring boot 版本在哪声明的？
2，提示插件找不到
Could not find org.springframework.boot:spring-boot-gradle-plugin:1.5.7.RELEASE.

3，spring boot 官方建议使用spring boot gradle 插件，需要构建脚本

在第一行加入
buildscript {
ext {
springBootVersion = '1.5.7.RELEASE'
}
repositories {
mavenCentral()
}
dependencies {
classpath("org.springframework.boot:spring-boot-gradle-plugin:${springBootVersion}")
}
}