Android config.gradle_apply from: 'config.gradle

对版本以及依赖控件版本进行统一管理：

1、创建config.gradle放在项目跟目录

ext{
    android=[
 
            compileSdkVersion       : 27,
            buildToolsVersion       : "27.0.3",
            minSdkVersion           : 15,
            targetSdkVersion        : 27,
            versionCode      : 1,
            versionName     : "1.0"
    ]
    versions = [
            retrofitSdkVersion      : "2.3.0",
    ]
    dependencies=[
            //support
            "constraint-layout":"com.android.support.constraint:constraint-layout:1.0.2",
            "appcompat-v7":"com.android.support:appcompat-v7:26.1.0",
            //net
            "retrofit":"com.