mkdir tmp instead of keys

[SalesforceRocks]

Hi,

Great work! There is however a little error I think in the circle.yml and there is some room for improvement.

Error:
You create a tmp dir. Shouldn't that be keys? The build fails otherwise.

Improvement:
Installing sfdx can be done now by the following (old lines commented):

#- wget -qO- $DX_CLI_URL | tar xJf -
#- ./sfdx/install
- npm install -g npm
- npm install --global sfdx-cli

The reason why you have to update npm is because CircleCI 1.0 has an old npm version installed. Before you can upgrade npm however you have to update node as well. So add this directly under the machine statement:

machine:
  node:
    version: 8.4.0