Remove gulp, and babel dependencies

[Jonathan Gros-Dubois]

These dependencies do very little (only to allow us to specify the order of tasks to build the client bundle). They introduce a large amount of sub-dependencies and they are a constant headache as they constantly raise vulnerability warnings over time.

It would be good to be able to build the client using only a simple Node.js script and browserify. No need for these clunky libraries which do almost nothing.