Only transpile for IE11 target in CI & production builds

no issue
- makes debugging during development easier because there's a lot less indirection added to work around missing ES6 support in IE11

ghost/admin/config/targets.js

@@ -1,11 +1,19 @@
 /* eslint-env node */
 
+const browsers = [
+    'last 2 Chrome versions',
+    'last 2 Firefox versions',
+    'last 2 Safari versions',
+    'last 2 Edge versions'
+];
+
+const isCI = !!process.env.CI;
+const isProduction = process.env.EMBER_ENV === 'production';
+
+if (isCI || isProduction) {
+    browsers.push('ie 11');
+}
+
 module.exports = {
-    browsers: [
-        'ie 11',
-        'last 2 Chrome versions',
-        'last 2 Firefox versions',
-        'last 2 Safari versions',
-        'last 2 Edge versions'
-    ]
+    browsers
 };