Error : Failed to find: com.android.support:support-v4:20.0.+

The Android support repository was missing,So go to Android SDK, install the Android Support Repository and Android Support Library. Also you can use the following

compile 'com.android.support:support-v4:20.0.0'
compile 'com.android.support:appcompat-v7:20.0.0'

instead of

compile 'com.android.support:appcompat-v7:20.0.+'
compile 'com.android.support:support-v4:20.0.+'

Note: If you're developing with Android Studio, select and install the Android Support Repository item instead.

see here.

From Android Studio go to: Tools >> Android >> SDK Manager

Select and install "Extras|Android Support Repository"