Allow 'en' in the list of supported locales

.travis.yml

@@ -1,12 +1,15 @@
 language: node_js
-before_install:
-# package-lock.json was introduced in npm@5
-- '[[ $(node -v) =~ ^v9.*$ ]] || npm install -g npm@latest' # skipped when using node 9
-- npm install -g greenkeeper-lockfile@1
-before_script: greenkeeper-lockfile-update
-after_script: greenkeeper-lockfile-upload
 jobs:
   include:
+      # ↓ The Greenkeeper job
+    - before_install:
+      # package-lock.json was introduced in npm@5
+      - '[[ $(node -v) =~ ^v9.*$ ]] || npm install -g npm@latest' # skipped when using node 9
+      - npm install -g greenkeeper-lockfile@1
+      script:
+      - greenkeeper-lockfile-update
+      - greenkeeper-lockfile-upload
+      node_js: "stable"
     - script: npm test
       node_js: "stable"
       # Check that the code is properly parsed in Node.js v4

index.js

@@ -48,13 +48,18 @@ function normalizeLocalesToKeep(localesToKeep) {
         );
     }
 
-    // Check if has unknown locales
+    // 'en' is built into Moment, so it doesn't exist in the locales context
+    localesToKeep = localesToKeep.filter(function(localeName) {
+        return localeName !== 'en';
+    });
+
+    // Check if it has unknown locales
     var absentLocales = localesToKeep.filter(function(localeName) {
-        const localeData = moment.localeData(localeName);
+        var localeData = moment.localeData(localeName);
         return (
             // For Moment 2.20.1−
             localeData === null ||
-            // For Moment. 2.21.0+ – this version now returns a default locale instead of null
+            // For Moment. 2.21.0+ – this version now returns the localeData of the currently set locale, instead of null
             localeData === moment.localeData()
         );
     });