Did you successfully build and install the SDK from the source (per the instructions here and here)?
If you had trouble doing so, or if you just want to use a pre-built copy, you can instead replace this line in the build.gradle:
compile group: 'com.dropbox.core', name: 'dropbox-core-sdk', version: '0-SNAPSHOT', changing: true
with:
compile 'com.dropbox.core:dropbox-core-sdk:2.1.2'