feat: improve deprecation notices during development

[jlengstorf]

This update tells developers the new name of the deprecated props to make it easier to upgrade.

re #3986

[jlengstorf]

Any ideas what caused this failure in Travis or how to fix it?

$ babel-node scripts/check-versions.js
gatsby-source-drupal: 
  Depends on "gatsby-source-filesystem@next" 
  instead of "gatsby-source-filesystem@2.0.0-alpha.8". 
gatsby-source-wordpress: 
  Depends on "gatsby-source-filesystem@next" 
  instead of "gatsby-source-filesystem@2.0.0-alpha.8". 

[jlengstorf]

This is what the updated errors look like, by the way.

[pieh]

I've caused check-version failure in Travis ( #5170 - context for that ), but test weren't passing either way on v2. Will look into fixing that particular thing tomorrow

[m-allanson]

Nice! Thanks @jlengstorf.