Add invariant for value returned when user modifies the Webpack config (#300)
Add invariant for value returned when user modifies the Webpack config (#300)

Fixes #269
0x80/gatsby,gatsbyjs/gatsby,Khaledgarbaya/gatsby,fabrictech/gatsby,0x80/gatsby,0x80/gatsby,chiedo/gatsby,rothfels/gatsby,rothfels/gatsby,ChristopherBiscardi/gatsby,Khaledgarbaya/gatsby,mickeyreiss/gatsby,ChristopherBiscardi/gatsby,mingaldrichgan/gatsby,gatsbyjs/gatsby,fabrictech/gatsby,okcoker/gatsby,chiedo/gatsby,fk/gatsby,ChristopherBiscardi/gatsby,fk/gatsby,okcoker/gatsby,mingaldrichgan/gatsby,gatsbyjs/gatsby,fk/gatsby,mingaldrichgan/gatsby,fabrictech/gatsby,mickeyreiss/gatsby,danielfarrell/gatsby,okcoker/gatsby,gatsbyjs/gatsby,mickeyreiss/gatsby,ChristopherBiscardi/gatsby,gatsbyjs/gatsby,chiedo/gatsby,Khaledgarbaya/gatsby,danielfarrell/gatsby,danielfarrell/gatsby,gatsbyjs/gatsby