fixed gitignore

Line 14 (img) caused the entire /dist/img directory to be ignored even though it is already in git, so new images were ignored. Prefixing it with / prevents this very general rule from applying to the subdirs. The same logic applies to the other rules for top level dirs.

.gitignore

@@ -1,17 +1,17 @@
 .DS_Store
-data/data.js
-dist/*.js
-dist/*.css
-dist/*.map
-dist/img/*.svg
-dist/mapillary-js/
+/data/data.js
+/dist/*.js
+/dist/*.css
+/dist/*.map
+/dist/img/*.svg
+/dist/mapillary-js/
 node_modules/
 npm-debug.log
 transifex.auth
 
 # autogenerated symlinks
 land.html
-img
-css/img
-test/css
-test/img
+/img
+/css/img
+/test/css
+/test/img