cz.prompt(...).then is not a function

Getting the following error on cz-conventional-changelog@1.1.6:

```
node_modules/cz-conventional-changelog/index.js:79
]).then(function(answers) {
^

TypeError: cz.prompt(...).then is not a function
at module.exports.prompter (node_modules/cz-conventional-changelog/index.js:79:8)
at commit (.nvm/versions/node/v5.10.1/lib/node_modules/commitizen/dist/commitizen/commit.js:68:5)
at .nvm/versions/node/v5.10.1/lib/node_modules/commitizen/dist/cli/strategies/git-cz.js:89:30
at .nvm/versions/node/v5.10.1/lib/node_modules/commitizen/dist/commitizen/staging.js:30:5
at .nvm/versions/node/v5.10.1/lib/node_modules/commitizen/node_modules/gulp-git/lib/exec.js:27:5
at ChildProcess.exithandler (child_process.js:193:7)
at emitTwo (events.js:100:13)
at ChildProcess.emit (events.js:185:7)
at maybeClose (internal/child_process.js:850:16)
at Socket. (internal/child_process.js:323:11)
```

v1.1.5 appears to work as expected.