Use space indentation for all YAML files in `.editorconfig`

YAML only accepts spaces for indentation, not tabs.

(cherry picked from commit 076b17ff6c1b0cec8be8c329a93ec0b5acf4d285)

.editorconfig

@@ -17,7 +17,8 @@ indent_size = 4
 indent_style = space
 indent_size = 4
 
-[.travis.yml]
+# YAML requires indentation with spaces instead of tabs.
+[*.{yml,yaml}]
 indent_style = space
 indent_size = 2