Error dependencies Gradle: all google.com.gms.Ibraries must use the Exact same version Specification (Mixing versions can lead to Runtime crashes)

Question

I am trying to add a barcode reader to my application and am using this example as the basis: https://github.com/KingsMentor/MobileVisionBarcodeScanner The error appears whenever I arrive at step 2, where it says I need to add a new dependency on Gradle:

dependencies {
        compile('com.github.KingsMentor:MobileVisionBarcodeScanner:v1.2') { transitive = true; }
}

When I tell you to sync, the line

compile 'com.google.android.gms:play-services:7.8.0'

is underlined red with the message

all google.com.gms.Libraries must use the Exact same version Specification (Mixing versions can lead to Runtime crashes). Found versions 9.4.0, 7.8.0. Examples include com.google.android.gms:play-services-Vision:9.4.0 and com.google.android.gms:play-services:7.8.0

and the error message appears on the Gradle console:

Error:Execution failed for task ':app:processDebugManifest'.

Manifest Merger failed : Attribute application@label value=() from Androidmanifest.xml:19:9-37 is also present at [com.github.Kingsmentor:Mobilevisionbarcodescanner:v1.2] Androidmanifest.xml:15:9-41 value=(@string/app_name). Suggestion: add 'tools:replace="android:label"' to element at Androidmanifest.xml:16:5-369:19 to override.

I already tried to add the line compile 'com.google.android.gms:play-services-vision:9.4.0' but the error continues