Try CircleCI memory+ class to fix build step (6GB instead of 4GB of memory)

[jeanregisser]

Description

Though #1549 built fine, merging it caused the initial install step to run out of memory:

https://app.circleci.com/jobs/github/celo-org/celo-monorepo/66762

This PR makes CircleCI run the failing step with a VM with more memory (6GB instead of the default 4GB).

[jmrossy]

Any theories why a smaller yarn.lock is now requiring more resources?

[jeanregisser]

Given more packages are now deduplicated, there are more packages in the root node_modules.
In this specific case the mobile build step consumed more memory apparently.
So my guess is TypeScript parsing more typings from the root node_modules/@types folder (by default it includes all typings there).

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (master@f5a8fc9). Click here to learn what that means.
The diff coverage is n/a.

@@            Coverage Diff            @@
##             master    #1770   +/-   ##
=========================================
  Coverage          ?   74.16%           
=========================================
  Files             ?      278           
  Lines             ?     7656           
  Branches          ?      674           
=========================================
  Hits              ?     5678           
  Misses            ?     1860           
  Partials          ?      118

Flag	Coverage Δ
#mobile	74.16% <ø> (?)

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