build.gradle.example

buildscript {
    repositories {
      ...
    }

    dependencies {
        classpath 'io.fabric.tools:gradle:1.+'
    }
}
apply plugin: 'com.android.application'
...

repositories {
    ...
}

def versionPropsFile = file('version.properties')

android {
    compileSdkVersion 25
    buildToolsVersion "25.0.2"

    if (versionPropsFile.canRead()) {
        def Properties versionProps = new Properties()
        versionProps.load(new FileInputStream(versionPropsFile))
        def code = versionProps['VERSION_CODE'].toInteger()
        def name = versionProps['VERSION_NAME']

        defaultConfig {
            applicationId "com.projectname"
            minSdkVersion 17
            targetSdkVersion 25
            versionCode code //1
            versionName name //1.0
            testInstrumentationRunner "android.support.test.runner.AndroidJUnitRunner"
        }
    }
    else {
        throw new GradleException("Could not read version.properties!")
    }

    productFlavors {
  ...
    }

    signingConfigs {
...
    }

    buildTypes {
  ...
    }
}

dependencies {
  ...
}

...

task('incrementBuildNumber') << {
    def code = 1

    if (versionPropsFile.canRead()) {
        def Properties versionProps = new Properties()

        versionProps.load(new FileInputStream(versionPropsFile))

        code = versionProps['VERSION_CODE'].toInteger() + 1

        versionProps['VERSION_CODE']=code.toString()
        versionProps.store(versionPropsFile.newWriter(), null)

    }  else {
        throw new GradleException("Could not read version.properties!")
    }
    return code
}