Ignore asdf .tool-versions file

asdf-vm uses a different file to other managers, this adds
.tool-versions to the ignore list, which is a file needed to manage the
Ruby version locally to this project.

.gitignore

@@ -17,6 +17,7 @@ eslint-report.html
 .rbx/
 /.ruby-gemset
 /.ruby-version
+/.tool-versions
 /.rvmrc
 .sass-cache/
 /.secret