https://docs.travis-ci.com/api#settings:-environment-variables
add(Travis.Client.t, list) :: Travis.response
delete_var(binary, Travis.Client.t, list) :: Travis.response
list(Travis.Client.t, list) :: Travis.response
show(binary, Travis.Client.t, list) :: Travis.response
update(binary, Travis.Client.t, list) :: Travis.response