Git-ignore npm-installed node modules

Without this tiny change, projects using semver as a Git submodule (any other way?) always get this message:

```bash
# Changes not staged for commit:
#   (use "git add <file>..." to update what will be committed)
#   (use "git checkout -- <file>..." to discard changes in working directory)
#   (commit or discard the untracked or modified content in submodules)
#
#	modified:   node_modules/semver (untracked content)
#
```