Auto increment version number on release builds... You can change the build type on versionCode.gradle, you also will need to commit and push gradle.properties on your CI

build.gradle

apply from: 'versionCode.gradle'

android {
    defaultConfig {
        versionName VERSION_NAME
        versionCode Integer.parseInt(VERSION_CODE)
    }
}

gradle.properties

VERSION_NAME=1.0.0
VERSION_CODE=10

versionCode.gradle

task('increaseVersionCode') << {
    def versionCode = Integer.parseInt(VERSION_CODE) + 1
    ant.propertyfile(file: "../gradle.properties") {
        entry(key: "VERSION_CODE", value: versionCode)
    }
}

tasks.whenTaskAdded { task ->
    if (task.name == 'generateReleaseBuildConfig) {
        task.dependsOn 'increaseVersionCode'
    }
}