Speedup the Continuous Documentation using mamba
* Simplify the vercel build commands
`gmt` is already listed as a dependency in
[environment.yml](https://github.com/GenericMappingTools/pygmt/blob/master/environment.yml),
so running `conda env create -f environment.yml` already has `gmt`
installed. No need to install `gmt` again.
* Use mamba
{
"scripts": {
"build:miniconda": "curl -o ~/miniconda.sh https://repo.anaconda.com/miniconda/Miniconda3-latest-Linux-x86_64.sh && bash ~/miniconda.sh -b -p $HOME/miniconda",
- "build:pygmt": "conda env create -f environment.yml && source activate pygmt && conda install -c conda-forge -y gmt==6.1.1 && make install",
+ "build:pygmt": "conda install mamba -c conda-forge -y && mamba env create -f environment.yml && source activate pygmt && make install",
"build:docs": "source activate pygmt && cd doc && make all && mv _build/html ../public",
"build": "export PATH=$HOME/miniconda/bin:$PATH && npm run build:miniconda && npm run build:pygmt && npm run build:docs"
}
