Fix JSON output of invocation transaction

[ixje]

What current issue(s) does this address, or what feature is it adding?
Fix #749

How did you solve this problem?
Fix return value of systemfee, change JSON print format of GAS key.
Note that fixing the system_fee results in fixing the net_fee, because
network_fee = input - output - SystemFee

How did you make sure your solution works?
make test and manually query tx to match output with neo-cli output shown in the issue.

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

Please check the following, if applicable:

• Did you add any tests?
• Did you run make lint?
• Did you run make test?
• Are you making a PR to a feature branch or development rather than master?
• Did you add an entry to CHANGELOG.rst? (if not, please do)

[coveralls]

Coverage remained the same at 83.969% when pulling 5744bfe on ixje:fix_getrawtx_output into 633095f on CityOfZion:development.