Pin [email protected] to fix npm-based install

[billagee]

I tried running npm run dev in a fresh install of the emilia starter on a system without yarn installed, and the command fails with this error:

success onPostBootstrap — 0.228 s
error Cannot find module 'core-js/modules/es6.regexp.to-string'

Running npm install --save-prod [email protected] seems to fix it. After doing that npm run dev with yarn installed still seems to succeed as well.

Looks like this same issue has popped up in other gatsby starters; so I basically borrowed one of their fixes:

maxpou/gatsby-starter-morning-dew#29

[LekoArts]

Hi, thank you for the PR! Since this is an issue with npm (Gatsby already has that as a dependency), it works fine with yarn I don't want to incorporate this change :/