Reduce dependencies

[thibautjombart]

Currently 113 packages are needed when installing treescape. We need to reduce the dependencies to a minimum.

[thibautjombart]

For instance:
https://github.com/thibautjombart/treescape/blob/master/R/servers.R#l14-l30

I don't think any of these are actually used in the corresponding functions.

[hyanwong]

In particular, it's a bit annoying that whenever you load treescape you get the message:

Creating a generic function for ‘toJSON’ from package ‘jsonlite’ in pack
age ‘googleVis’

I wonder how vital googleVis is to most treescape functions?