Fix Attribute class naming so methods work in VM

[slipo]

What current issue(s) from Github does this address?
None AFAIK

What problem does this PR solve?
The TransactionAttribute class wasn't working. I was running code like this:

from boa.blockchain.vm.Neo. TransactionAttribute import GetData
...
attributes = tx.Attributes
attribute = attributes[0]
sending_script_hash = GetData(attribute)
# Error here!
print(sending_script_hash)

Error was:

[I 180126 00:11:53 InteropService:304] method Neo.TransactionAttribute.GetData not found in ->
[I 180126 00:11:53 InteropService:306] Neo.Asset.GetOwner -> <bound method StateReader.Asset_GetOwner of <neo.SmartContract.StateReader.StateReader object at 0x111c05ef0>>
[I 180126 00:11:53 InteropService:306] AntShares.Header.GetNextConsensus -> <bound method StateReader.Header_GetNextConsensus of <neo.SmartContract.StateReader.StateReader object at 0x111c05ef0>>

How did you solve this problem?
Renamed the class to match what the VM expects.

How did you make sure your solution works?
Now my code works and getData returns the data I expect.

Are there any special changes in the code that we should be aware of?
No

Is there anything else we should know?
No

[coveralls]

Coverage remained the same at 80.09% when pulling 79879cc on slipo:development into 4ba216d on CityOfZion:development.

[localhuman]

Great bug find and fix. Thank you! 🥇