Error dependencies gradle: all google.com.gms.libraries must use the exact same version specification (mixing versions can lead to runtime crashes)

I'm trying to add a barcode reader to my application and I'm using this example as the basis: link The error appears whenever I get to step 2, where it says that I need to add a new dependency in the gradle:

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

When you sync, the

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

Underlined red with 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 in 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 have tried to add the line compile 'com.google.android.gms:play-services-vision:9.4.0' but the error continues