Update build.gradle
Update build.gradle

I have updated the 26.+ to the correct version. And yes the Gradle version also has to be upgraded in the file AndroidSlidingUpPanel/build.gradle
Line no 8 to -> classpath 'com.android.tools.build:gradle:2.3.2'
and
Line no 26 to -> gradleVersion = '3.3'
hannesa2/AndroidSlidingUpPanel,umano/AndroidSlidingUpPanel,hannesa2/AndroidSlidingUpPanel