update requeueable exceptions delay policy algorithm

[vadim-ex]

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

[dlecocq]

Rebased off of master and force-pushed (sorry about that). It's to incorporate a fix for a flaky test.

[dlecocq]

Once this build passes, LGTM

[vadim-ex]

Thank you Dan! I really appreciate doing rebase for me