Replace sprockets-es6 with babel-transpiler

Sprockets 4 now has direct babel support, so we can remove the temporary
sprockets-es6 dependency and use babel-transpiler directly.

Gemfile

@@ -87,12 +87,12 @@ gem "uuid", "2.3.9"
 
 # JavaScript
 
+gem "babel-transpiler",  "0.7.0"
 gem "handlebars_assets", "0.23.9"
 gem "jquery-rails",      "4.5.0"
 gem "jquery-ui-rails",   "5.0.5"
 gem "js_image_paths",    "0.2.0"
 gem "js-routes",         "2.2.4"
-gem "sprockets-es6",     "0.9.2"
 
 # Localization
 

Gemfile.lock

@@ -679,10 +679,6 @@ GEM
     sprockets (4.1.1)
       concurrent-ruby (~> 1.0)
       rack (> 1, < 3)
-    sprockets-es6 (0.9.2)
-      babel-source (>= 5.8.11)
-      babel-transpiler
-      sprockets (>= 3.0.0)
     sprockets-rails (3.4.2)
       actionpack (>= 5.2)
       activesupport (>= 5.2)
@@ -780,6 +776,7 @@ DEPENDENCIES
   apparition (= 0.6.0)
   asset_sync (= 2.15.2)
   autoprefixer-rails (= 10.4.7.0)
+  babel-transpiler (= 0.7.0)
   bootstrap-sass (= 3.4.1)
   bootstrap-switch-rails (= 3.3.3)
   capybara (= 3.35.3)
@@ -868,7 +865,6 @@ DEPENDENCIES
   simple_captcha2 (= 0.5.0)
   simplecov (= 0.21.2)
   sinon-rails (= 1.15.0)
-  sprockets-es6 (= 0.9.2)
   sprockets-rails (= 3.4.2)
   string-direction (= 1.2.2)
   terser (= 1.1.10)