HttpClient is deprecated in sdk 23.
You have to move on URLConnection or down sdk to 22
Still you need HttpClient with update gradle sdk 23
You have to add the dependencies of HttpClient in app/gradle as
dependencies {
compile fileTree(include: ['*.jar'], dir: 'libs')
compile 'com.android.support:appcompat-v7:23.0.1'
compile 'org.jbundle.util.osgi.wrapped:org.jbundle.util.osgi.wrapped.org.apache.http.client:4.1.2'
...
}