App not working on smaller versions

Question

I have an app with has minSdkVersion 15 ( 4.0.something ) and when I run my app on my phone (4.4.something (KitKat) ) App crashes on start but when I run it on higher versions like 5.0 It starts the app ?

Android Manifest :

apply plugin: 'com.android.application'

android {
    compileSdkVersion 26
    defaultConfig {
        applicationId "com.bdpsoft.bitcoinfaucet"
        minSdkVersion 15
        targetSdkVersion 26
        versionCode 1
        versionName "1.0"
        testInstrumentationRunner "android.support.test.runner.AndroidJUnitRunner"
        multiDexEnabled true
    }

    packagingOptions {
        exclude 'META-INF/NOTICE' // will not include NOTICE file
        exclude 'META-INF/LICENSE' // will not include LICENSE file
        // as noted by @Vishnuvathsan you may also need to include
        // variations on the file name. It depends on your dependencies.
        // Some other common variations on notice and license file names
        exclude 'META-INF/notice'
        exclude 'META-INF/notice.txt'
        exclude 'META-INF/license'
        exclude 'META-INF/license.txt'
    }


    buildTypes {
        release {
            minifyEnabled false
            proguardFiles getDefaultProguardFile('proguard-android.txt'), 'proguard-rules.pro'
        }
    }
}

dependencies {
    implementation fileTree(dir: 'libs', include: ['*.jar'])
    compile 'com.google.firebase:firebase-core:11.0.2'
    compile 'com.google.firebase:firebase-auth:11.0.2'
    compile 'com.google.firebase:firebase-database:11.0.2'
    compile 'com.google.firebase:firebase-crash:11.0.2'
    compile 'com.google.android.gms:play-services-auth:11.0.2'
    compile 'com.google.android.gms:play-services-ads:11.0.2'
    compile 'com.google.android.gms:play-services:11.0.2'
    compile 'com.google.firebase:firebase-messaging:11.0.2'
    compile 'com.firebase:firebase-client-android:2.3.1'
    compile 'com.github.ybq:Android-SpinKit:1.1.0'
    compile 'com.android.support:multidex:1.0.2'
    compile 'com.github.traex.rippleeffect:library:1.3'
    //noinspection GradleCompatible
    implementation 'com.android.support:appcompat-v7:27.0.2'
    implementation 'com.android.support.constraint:constraint-layout:1.0.2'
    testImplementation 'junit:junit:4.12'
    androidTestImplementation 'com.android.support.test:runner:1.0.1'
    androidTestImplementation 'com.android.support.test.espresso:espresso-core:3.0.1'
}

apply plugin: 'com.google.gms.google-services'

I dont know what is the problem , I tried to lover the minSdkVersion but it is not helping.