Set minimum browser version in manifest

Set the minimum browser version supported in the extension manifest.
Currently we only ship the extension on Chrome and Firefox, so the
minimum version has been set for those two browsers.

Relates to #6805

app/manifest.json

@@ -21,7 +21,8 @@
   },
   "applications": {
     "gecko": {
-      "id": "[email protected]"
+      "id": "[email protected]",
+      "strict_min_version": "53.0"
     }
   },
   "default_locale": "en",
@@ -85,4 +86,4 @@
       "*"
     ]
   }
-}
+}

gulpfile.js

@@ -153,6 +153,7 @@ gulp.task('manifest:chrome', function () {
   return gulp.src('./dist/chrome/manifest.json')
   .pipe(jsoneditor(function (json) {
     delete json.applications
+    json.minimum_chrome_version = '58'
     return json
   }))
   .pipe(gulp.dest('./dist/chrome', { overwrite: true }))