Install the latest version of Yarn when building sites (#1993)
Install the latest version of Yarn when building sites (#1993)

* Install the latest version of Yarn when building sites

* Run bootstrap before building

Khaledgarbaya/gatsby,mickeyreiss/gatsby,fk/gatsby,mingaldrichgan/gatsby,ChristopherBiscardi/gatsby,danielfarrell/gatsby,fk/gatsby,mingaldrichgan/gatsby,gatsbyjs/gatsby,mingaldrichgan/gatsby,fabrictech/gatsby,ChristopherBiscardi/gatsby,Khaledgarbaya/gatsby,gatsbyjs/gatsby,gatsbyjs/gatsby,gatsbyjs/gatsby,0x80/gatsby,gatsbyjs/gatsby,danielfarrell/gatsby,0x80/gatsby,0x80/gatsby,ChristopherBiscardi/gatsby,mickeyreiss/gatsby,ChristopherBiscardi/gatsby,Khaledgarbaya/gatsby,gatsbyjs/gatsby,fk/gatsby,danielfarrell/gatsby,mickeyreiss/gatsby,fabrictech/gatsby,fabrictech/gatsby