Increment minimum Firefox version required

Several manifest.json keys related to the options page and the
runtime.openOptionsPage method are only supported in Firefox for Android
since version 57.0.

Therefore, the minimum version has been incremented from Firefox 55.0 to
Firefox 57.0.

Change-Id: Ideaef5dda90b5e053da8dc1b8121b9e51596cd56
diff --git a/templates/manifest.gjson b/templates/manifest.gjson
index d414749..f6b8d5f 100644
--- a/templates/manifest.gjson
+++ b/templates/manifest.gjson
@@ -81,7 +81,7 @@
   "browser_specific_settings": {
     "gecko": {
       "id": "twpowertools@avm99963.com",
-      "strict_min_version": "55.0"
+      "strict_min_version": "57.0"
     }
   },
 #endif