Fixed info about usage with rspec + add section about bundler problems with aruba

Hi,

I think one would like to clean the current directory before each run to make sure you've a got clean testing environment. I added this section to the readme.

I was a bit lazy to open another PR for the second part I added to the README. I faced problems with aruba when using `bundler`. I thought my solution might be helpful for others as well.

/dg