はじめに

Gradle5.0から正式にBOM(bill of materials)ファイルの読み込みに対応しました。
まだ提供されているライブラリは多くないですが、このBOMファイルによってライブラリ毎のバージョン定義を省くことができるようになります。
参考: https://docs.gradle.org/5.0/userguide/managing_transitive_dependencies.html#sec:bom_import

Before


dependencies {

    // Kotlin
    implementation "org.jetbrains.kotlin:kotlin-stdlib-jdk8:$version_kotlin"
    implementation "org.jetbrains.kotlin:kotlin-test-junit5:$version_kotlin"

    // Firebase
    implementation 'com.google.firebase:firebase-core:16.0.8'
    implementation 'com.google.firebase:firebase-storage:16.1.0'
    implementation 'com.google.firebase:firebase-config:16.4.1'
    implementation 'com.google.firebase:firebase-invites:16.1.1'
    implementation 'com.google.firebase:firebase-messaging:17.5.0'
    implementation 'com.google.firebase:firebase-firestore:18.2.0'
    implementation 'com.google.firebase:firebase-dynamic-links:16.1.8'
    implementation 'com.google.firebase:firebase-perf:16.2.4'
    implementation 'com.crashlytics.sdk.android:crashlytics:2.9.3'

    // OkHttp
    def version_okhttp = '3.12.2'
    implementation "com.squareup.okhttp3:okhttp:$version_okhttp"
    implementation "com.squareup.okhttp3:logging-interceptor:$version_okhttp"
    testImplementation "com.squareup.okhttp3:mockwebserver:$version_okhttp"
}

After

dependencies {

    // kotlin
    implementation platform('org.jetbrains.kotlin:kotlin-bom:1.3.61')
    implementation 'org.jetbrains.kotlin:kotlin-stdlib-jdk8'
    implementation 'org.jetbrains.kotlin:kotlin-test-junit5'

    // Firebase
    implementation platform('com.google.firebase:firebase-bom:16.0.0')
    implementation 'com.google.firebase:firebase-core'
    implementation 'com.google.firebase:firebase-storage'
    implementation 'com.google.firebase:firebase-config'
    implementation 'com.google.firebase:firebase-invites'
    implementation 'com.google.firebase:firebase-messaging'
    implementation 'com.google.firebase:firebase-firestore'
    implementation 'com.google.firebase:firebase-dynamic-links'
    implementation 'com.google.firebase:firebase-perf'
    implementation 'com.crashlytics.sdk.android:crashlytics'

    // OkHttp
    implementation platform('com.squareup.okhttp3:okhttp-bom:3.12.2')
    implementation 'com.squareup.okhttp3:okhttp'
    implementation 'com.squareup.okhttp3:logging-interceptor'
    testImplementation 'com.squareup.okhttp3:mockwebserver'
}

Kotlin Libraries Bill of Materials: https://mvnrepository.com/artifact/org.jetbrains.kotlin/kotlin-bom
Firebase BOM: https://mvnrepository.com/artifact/com.google.firebase/firebase-bom
OkHttp (Bill of Materials): https://mvnrepository.com/artifact/com.squareup.okhttp3/okhttp-bom