Reorganize requirements.

Stab at issue #194

Have requirements.txt use setup.py, this way there is only one place of
requirements for users.
DescribeInstances: Added block_device_mapping to XML
Merge pull request #205 from DreadPirateShawn/DescribeInstancesBlockDeviceMapping

DescribeInstances: Added block_device_mapping to XML
Merge pull request #203 from IlyaSukhanov/master

Reorganize requirements.
Travis needs dev requirements.