Move yarn installation into before_install scripts
Move yarn installation into before_install scripts

refs #9838

For some reason Travis was pulling in a different key, and so the yarn
package could not be verified. The apt addon for travis displays that it
is in beta - so for now the installation of yarn has been moved into
before_install which should be more stable.

ErisDS/Ghost,Kikobeats/Ghost,e10/Ghost,bitjson/Ghost,kevinansfield/Ghost,e10/Ghost,kevinansfield/Ghost,TryGhost/Ghost,bitjson/Ghost,JohnONolan/Ghost,TryGhost/Ghost,janvt/Ghost,Kikobeats/Ghost,JohnONolan/Ghost,Gargol/Ghost,Gargol/Ghost,JohnONolan/Ghost,ErisDS/Ghost,kevinansfield/Ghost,TryGhost/Ghost,Kikobeats/Ghost,janvt/Ghost,ErisDS/Ghost