Skip DB related tasks if nothing has changed since the last deploy

This is based on http://blog.carbonfive.com/2013/08/12/minimizing-downtime-when-deploying-to-heroku/ and adapted to our needs. Here's "da numbers" http://pastebin.com/BpMhJ2Y1

Those numbers come from a brand new `pah` generated app with a couple of dummy models using the generator built from sources (it's a Rails 4.0.2 app with Ruby 2.1). I also got the numbers from a second `rake integrate` in a row, meaning the application has already been deployed and installing gems / compiling assets, etc.... does not impact on the overall numbers.

As you can see on the gist, I was able to lower it from `1m13.418s` down to `0m59.384s` (around 14 seconds per `rake integrate`). That seems a small change to what I expected from my numbers on GH-18 but the reason behind that is because the application is kinda "blank". I tracked that number down to spinning up the rails environment for the tasks to run (aka `rake environment`) and compared to the numbers I got from the original issue.

From this dummy app:

```
time \heroku run rake environment
Running `rake environment` attached to terminal... up, run.6385

real	0m7.790s
user	0m0.216s
sys	0m0.056s
```

And from the AS project:

```
 time \heroku run rake environment
Running `rake environment` attached to terminal... up, run.4424
[deprecated] I18n.enforce_available_locales will default to true in the future. If you really want to skip validation of your locale you can set I18n.enforce_available_locales = false to avoid this message.

real	0m15.380s
user	0m0.232s
sys	0m0.044s
```

In other words, as the project evolves and adds more dependencies, we'll save more time on `rake integrate`s