Added ability to change sbt tasks

We can now change the sbt tasks called by the buildpack by setting the environment var SBT_TASKS to whatever you want.

Example usage : 
   heroku config:set SBT_TASKS="'project myvariant' stage"
   git push heroku master

Should trigger the build on the project "myvariant"
Syntax error

I am always forgetting to add put a fi to close a if
Quoted sbt tasks

It seems to work better on my station with quotes
Merge pull request #1 from CedricGatay/patch-1

Added ability to change sbt tasks