remove nodejs_buildpack to prevent duplicate node_modules

The nodejs buildpack was creating a duplicate copy of node_modules
and wasting disk space, it is no longer needed after this update
to the ruby buildpack cloudfoundry/ruby-buildpack#373
was merged.

templates/manifest.yml.tt

@@ -2,7 +2,6 @@
 applications:
 - name: <%= app_name %>-((env))
   buildpacks:
-    - nodejs_buildpack
     - ruby_buildpack
   env:
     RAILS_MASTER_KEY: ((rails_master_key))