Railtie fails on case-sensitive file system

Got one more bug during `rake db:migrate` deploy to linux host

```
  * executing "cd /var/www/palguru/releases/20131116174002 && bundle exec rake RAILS_ENV=production  db:migrate"
    servers: ["prod.palguru.com"]
    [prod.palguru.com:20220] executing command
 ** [out :: prod.palguru.com:20220] rake aborted!
 ** [out :: prod.palguru.com:20220] 
 ** [out :: prod.palguru.com:20220] cannot load such file -- puff/Instrumentation/redis
 ** [out :: prod.palguru.com:20220] /var/www/palguru/shared/bundle/ruby/1.9.1/gems/activesupport-3.2.12/lib/active_support/dependencies.rb:251:in `require'
 ** [out :: prod.palguru.com:20220] /var/www/palguru/shared/bundle/ruby/1.9.1/gems/activesupport-3.2.12/lib/active_support/dependencies.rb:251:in `block in require'
 ** [out :: prod.palguru.com:20220] /var/www/palguru/shared/bundle/ruby/1.9.1/gems/activesupport-3.2.12/lib/active_support/dependencies.rb:236:in `load_dependency'
 ** [out :: prod.palguru.com:20220] /var/www/palguru/shared/bundle/ruby/1.9.1/gems/activesupport-3.2.12/lib/active_support/dependencies.rb:251:in `require'
 ** [out :: prod.palguru.com:20220] /var/www/palguru/shared/bundle/ruby/1.9.1/bundler/gems/puff-33e78689415b/lib/puff/instrumentation/log_subscriber.rb:1:in `<top (required)>'
 ** [out :: prod.palguru.com:20220] /var/www/palguru/shared/bundle/ruby/1.9.1/gems/activesupport-3.2.12/lib/active_support/dependencies.rb:251:in `require'
 ** [out :: prod.palguru.com:20220] /var/www/palguru/shared/bundle/ruby/1.9.1/gems/activesupport-3.2.12/lib/active_support/dependencies.rb:251:in `block in require'
 ** [out :: prod.palguru.com:20220] /var/www/palguru/shared/bundle/ruby/1.9.1/gems/activesupport-3.2.12/lib/active_support/dependencies.rb:236:in `load_dependency'
 ** [out :: prod.palguru.com:20220] /var/www/palguru/shared/bundle/ruby/1.9.1/gems/activesupport-3.2.12/lib/active_support/dependencies.rb:251:in `require'
 ** [out :: prod.palguru.com:20220] /var/www/palguru/shared/bundle/ruby/1.9.1/bundler/gems/puff-33e78689415b/lib/puff/engine.rb:1:in `<top (required)>'
 ** [out :: prod.palguru.com:20220] /var/www/palguru/shared/bundle/ruby/1.9.1/gems/activesupport-3.2.12/lib/active_support/dependencies.rb:251:in `require'
 ** [out :: prod.palguru.com:20220] /var/www/palguru/shared/bundle/ruby/1.9.1/gems/activesupport-3.2.12/lib/active_support/dependencies.rb:251:in `block in require'
 ** [out :: prod.palguru.com:20220] /var/www/palguru/shared/bundle/ruby/1.9.1/gems/activesupport-3.2.12/lib/active_support/dependencies.rb:236:in `load_dependency'
 ** [out :: prod.palguru.com:20220] /var/www/palguru/shared/bundle/ruby/1.9.1/gems/activesupport-3.2.12/lib/active_support/dependencies.rb:251:in `require'
 ** [out :: prod.palguru.com:20220] /var/www/palguru/shared/bundle/ruby/1.9.1/bundler/gems/puff-33e78689415b/lib/puff.rb:5:in `<top (required)>'
 ** [out :: prod.palguru.com:20220] /var/lib/gems/1.9.1/gems/bundler-1.3.5/lib/bundler/runtime.rb:72:in `require'
 ** [out :: prod.palguru.com:20220] /var/lib/gems/1.9.1/gems/bundler-1.3.5/lib/bundler/runtime.rb:72:in `block (2 levels) in require'
```