Bugfix to allow for configured babel presets such as the env preset.

[4rkanoid]

I tried using the babel-preset-env in my gatsby project and the babel-config module threw this exception:

Invariant Violation:
  You are trying to use a Babel plugin which Gatsby cannot find. You
  can install it using "npm install --save babel-preset-env,[object Object]".

  You can use any of the Gatsby provided plugins without installing them:
    - babel-plugin-add-module-exports
    - babel-plugin-transform-object-assign
    - babel-preset-es2015
    - babel-preset-react
    - babel-preset-stage-0

I figured it must be due to how the .babelrc is parsed and saw that there is no logic for extracting the preset name when the preset is in array form (which is done with plugins).

This pr has a simple fix that just replicate the logic for plugin names into doing the same thing for presets.
Cheers!

[KyleAMathews]

Oh nice! Thanks for the bug fix! Will push out in a release in a sec.

[4rkanoid]

Sweet 👍

[jlengstorf]

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