Exclude codemirror

An example of a wrong detection due to codemirror is my project.  https://github.com/mgaitan/waliki
Adding test for codemirror
Merge pull request #1743 from github/codemirror

Codemirror should be considered vendored
Rename file to avoid case-insensitive collision
Merge pull request #1745 from github/f-case

Fix failures on case-insensitive filesystem
Update grammars.yml
Bump version to v4.0.2
Merge pull request #1747 from github/cut-release-v4.0.2

Cut release v4.0.2
sync cached gems