[rails] Use `bin/rails` to start server (#10046)

Calling `rails` directly is deprecated and results in errors on newer
versions:

    3.5.0+0/rubygems.rb:269:in 'Gem.find_spec_for_exe': can't find gem
    railties (>= 0.a) with executable rails (Gem::GemNotFoundException)

frameworks/Ruby/rails/rails-mysql.dockerfile

@@ -25,4 +25,4 @@ ENV RAILS_ENV=production_mysql
 ENV PORT=8080
 ENV REDIS_URL=redis://localhost:6379/0
 CMD service redis-server start && \
-    rails server
+    bin/rails server

frameworks/Ruby/rails/rails.dockerfile

@@ -25,4 +25,4 @@ ENV RAILS_ENV=production_postgresql
 ENV PORT=8080
 ENV REDIS_URL=redis://localhost:6379/0
 CMD service redis-server start && \
-    rails server
+    bin/rails server