update requeueable exceptions delay policy algorithm

Change requeueable exception delay algorithm to generate random delay in the range (including fractional seconds). I think, the better way would be to separate requeue policy into its own class from requeue handling (this would allow multiple policies, like exponential extension of the delay, etc.), but it is more radical change, which would be breaking. OTOH, I am willing to implement it.

minor improvements:
+ fixed phantomjs to download 64-bit _linux_ image
+ added phantomjs/ to .gitignore