The 01.org server seems to be down. So till the servers are back online, you can ask Gradle to use the local copy of Crosswalk.
I was having the same issue from last one hour. The following solution worked for me.
Open platforms\android\cordova-plugin-crosswalk-webview\*-xwalk.gradle file in your project directory, where * is your project name.
Look for the section -
dependencies {
compile xwalkSpec
}
Replace it with your exact Crosswalk version which you find in Chrome Developer Tools by typing window.navigator.userAgent in your Developer Tools console.
So your final data would look something like this -
dependencies {
compile 'org.xwalk:xwalk_core_library:23.53.589.4'
}