build.gradle

android {
...
    def versionFile = file('version.properties')
    Properties prop = new Properties()
    if (versionFile.canRead()) {
        prop.load(new FileInputStream(versionFile))
    }
    else {
        prop['VERSION_CODE']='0'
    }
    def code = prop['VERSION_CODE'].toInteger() + 1
    prop['VERSION_CODE']=code.toString()
    prop.store(versionFile.newWriter(), null)

    defaultConfig {
        ...
        versionCode code
        versionName "1.0"
    }
...
}