Use readlink to get the absolute path of the buildpack

As we are using the buildpack elsewhere, the use of `readlink` results to be less 'herokuish'.

Here is a heroku deployment with the modification, it works just well.


```
└> git push heroku master   
Counting objects: 31, done.
Delta compression using up to 4 threads.
Compressing objects: 100% (16/16), done.
Writing objects: 100% (31/31), 3.27 KiB | 0 bytes/s, done.
Total 31 (delta 7), reused 31 (delta 7)

-----> Fetching custom git buildpack... done
-----> Go app detected
-----> Installing Go 1.1.2... done
       Installing Virtualenv... done
       Installing Mercurial... done
       Installing Bazaar... done
-----> Running: go get -tags heroku ./...
-----> Discovering process types
       Procfile declares types -> web, worker

-----> Compiled slug size: 1.6MB
-----> Launching... done, v4
       http://go-test-app-soulou.herokuapp.com deployed to Heroku

To git@heroku.com:go-test-app-soulou.git
 * [new branch]      master -> master
```