Add logs/ to be ignored by git

[Boorclark]

Description

This PR is a simple one-line change that will make the development process mildly easier by making sure that Git does not track build files.

Testing

1. Build locally using the command: python3 -m pretext build web.
2. Confirm that your build log remains accessible in the "logs/" directory.
3. Execute a git status command and verify that the "logs/" folder does not appear among the untracked items.

fixes #262

[pearcej]

good catch!