Clear node_modules and yarn.lock to fix some problems

[ehowey]

Describe the bug
Because of the number of dependencies managed by gatsby-theme-catalyst there are occasionally conflicts in yarn.lock or in node_modules that can be resolved by removing both of these folders/files and running yarn install.

This is a good first step if you are encountering an error with missing packages or version mismatch. Most often I have seen this as a visual error with Theme-UI and/or an error with different conflicting versions of Gatsby installed.

It is worth trying this before posting a bug report if you think it may solve your problem.

rm -rf node_modules yarn.lock
yarn install