Stop running mjml with yarn. Instead fetch the bin

[alanhala]

Instead of running the bin through yarn (yarn run mjml), it makes it work like npm which is a faster way.

[andrelaszlo]

This makes calling mjml ~40% faster on my machine.

[sighmon]

@alanhala Great thanks! Looks good to me and brings it inline with the logic in check_for_npm_mjml_binary.

I don't use yarn, so might check with @doits or @thechrisoshow who previously submitted PRs.

[sighmon]

Haven't heard back, so let's merge this and revert if need be.