fix failureMessage display

[jehy]

On my project I had a trouble with displaying test errors - instead of error text, some kind of array was logged:

I looked through the code and saw that failureMessage seems to be supposed a string, not an array. So I simply put a join('') there - and now everything seems to work fine.

I checked this on several differents reporters and results were same.
May be jest accepted failureMessage as an array before, bit now it expects string.
I suppose that this should be checked on other project setups as well.