Upgrade to Rails 4.2

* Update boilerplate with `bin/rake rails:upgrade`
* Use test instead of test:all rake task
* Change stylesheet extensions from .css.scss to .scss
* Update docs to use localhost instead of 0.0.0.0 because Rails 4.2
  no longer listens on all addresses by default.

app/assets/javascripts/application.js

@@ -2,7 +2,7 @@
 // listed below.
 //
 // Any JavaScript/Coffee file within this directory, lib/assets/javascripts, vendor/assets/javascripts,
-// or vendor/assets/javascripts of plugins, if any, can be referenced here using a relative path.
+// or any plugin's vendor/assets/javascripts directory can be referenced here using a relative path.
 //
 // It's not advisable to add code directly here, but if you do, it'll appear at the bottom of the
 // compiled file.

app/assets/stylesheets/application.scss

@@ -3,7 +3,7 @@
  * listed below.
  *
  * Any CSS and SCSS file within this directory, lib/assets/stylesheets, vendor/assets/stylesheets,
- * or vendor/assets/stylesheets of plugins, if any, can be referenced here using a relative path.
+ * or any plugin's vendor/assets/stylesheets directory can be referenced here using a relative path.
  *
  * You're free to add application-wide styles to this file and they'll appear at the bottom of the
  * compiled file so the styles you add here take precedence over styles defined in any styles