Unity does not support resolving dependences from a git url. Because of that, you will need to add the following lines to your manifest.json.
"dependencies": {
"com.juce.utils": "git+https://github.com/Juce-Assets/Juce-Utils",
"com.juce.core": "git+https://github.com/Juce-Assets/Juce-Core"
},