Change vue dep to be lower bound

[robinp]

Previously it was fixed version, pulling in duplicate Vue into the user project potentially.

Noticed by running the webpack-bundle-analyzer plugin (see https://dev.to/m9hmood/reduce-vue-js-application-size-kfl), then did npm ls vue, which printed

$ npm ls vue                                                                                                                                               
my-ui
├── vue@2.6.9
├─┬ vue-backtotop@1.6.1                                                                                                                                                                                              
  └── vue@2.3.3                                                                                                                                                                                                    ```
Thanks for the component btw!

[robinp]

Gentle ping.