Installs an ancient version of yarn by default

[jbrowning]

The default config specifies an ancient version of yarn to install by default:

yarn_version=0.27.5

This should instead be unset to allow for the latest version of yarn to be installed.

[nelsonic]

On the basis that Node.js ships with npm and there is zero benefit to using Yarn on Heroku,
could the yarn_version be removed completely? 🤔

[jesseshieh]

I think removing it completely is a good idea, but we have to worry about backward compatibility. Perhaps, we can start printing a warning and then remove it 6 months later. What do you think?
Also see #113

[jbrowning]

I may be misunderstanding but I strongly disagree that there is "zero benefit to using Yarn on Heroku" -- some projects use yarn to lock, others use npm to lock, and Heroku supports both.

@jesseshieh Is your proposal simply to remove the default version of yarn so that it must be specified in the buildpack settings?

[jesseshieh]

Sorry yes, my proposal is just to remove the default version.