Revert renaming tracking to letterSpacing

defaultConfig.stub.js

@@ -52,7 +52,7 @@ module.exports = {
     textTransform: ['responsive', 'hover', 'focus'],
     textDecoration: ['responsive', 'hover', 'focus'],
     fontSmoothing: ['responsive', 'hover', 'focus'],
-    letterSpacing: ['responsive'],
+    tracking: ['responsive'],
     userSelect: ['responsive'],
     verticalAlign: ['responsive'],
     visibility: ['responsive'],

defaultTheme.js

@@ -148,7 +148,7 @@ module.exports = function() {
       normal: 1.5,
       loose: 2,
     },
-    letterSpacing: {
+    tracking: {
       tight: '-0.05em',
       normal: '0',
       wide: '0.05em',

plugins/letterSpacing.js

@@ -1 +0,0 @@
-module.exports = require('../lib/plugins/letterSpacing').default

plugins/tracking.js

@@ -0,0 +1 @@
+module.exports = require('../lib/plugins/tracking').default

src/corePlugins.js

@@ -45,7 +45,7 @@ import fontStyle from './plugins/fontStyle'
 import textTransform from './plugins/textTransform'
 import textDecoration from './plugins/textDecoration'
 import fontSmoothing from './plugins/fontSmoothing'
-import letterSpacing from './plugins/letterSpacing'
+import tracking from './plugins/tracking'
 import userSelect from './plugins/userSelect'
 import verticalAlign from './plugins/verticalAlign'
 import visibility from './plugins/visibility'
@@ -119,7 +119,7 @@ export default function(config) {
     textTransform,
     textDecoration,
     fontSmoothing,
-    letterSpacing,
+    tracking,
     userSelect,
     verticalAlign,
     visibility,