New road colours and simplify text in key

Update colours for main roads to match
gravitystorm/openstreetmap-carto#1736 and replace the OSM keywords
trunk, primary and secondary (which have different meanings from OSM use
in some places) with a new keyword main_road (main road),

config/key.yml

@@ -1,11 +1,9 @@
 mapnik:
   - { min_zoom: 0, max_zoom: 19, name: motorway, image: motorway.png }
-  - { min_zoom: 0, max_zoom: 11, name: trunk, image: trunk.png }
-  - { min_zoom: 12, max_zoom: 19, name: trunk, image: trunk12.png }
-  - { min_zoom: 7, max_zoom: 11, name: primary, image: primary.png }
-  - { min_zoom: 12, max_zoom: 19, name: primary, image: primary12.png }
-  - { min_zoom: 9, max_zoom: 11, name: secondary, image: secondary.png }
-  - { min_zoom: 12, max_zoom: 19, name: secondary, image: secondary12.png }
+  - { min_zoom: 0, max_zoom: 6, name: main_road, image: mainroad.png }
+  - { min_zoom: 7, max_zoom: 8, name: main_road, image: mainroad7.png }
+  - { min_zoom: 9, max_zoom: 11, name: main_road, image: mainroad9.png }
+  - { min_zoom: 12, max_zoom: 19, name: main_road, image: mainroad12.png }
   - { min_zoom: 13, max_zoom: 19, name: track, image: track.png }
   - { min_zoom: 13, max_zoom: 19, name: bridleway, image: bridleway.png }
   - { min_zoom: 13, max_zoom: 19, name: cycleway, image: cycleway.png }

config/locales/en.yml

@@ -1,4 +1,4 @@
-en:
+en:
   html:
     dir: ltr
   time:
@@ -529,6 +529,7 @@ en:
           footway: "Footpath"
           ford: "Ford"
           living_street: "Living Street"
+          main_road: "Main Road"
           milestone: "Milestone"
           motorway: "Motorway"
           motorway_junction: "Motorway Junction"