Fix abi returntype casing

[EdgeDLT]

What current issue(s) from Github does this address?
Fix #135

What problem does this PR solve?
Fixes incorrect casing in .abi.json output so that it matches the Neon compiler output, improving compatibility with the Neo Blockchain Toolkit's Visual DevTracker.

How did you solve this problem?
Changed a big T to a little t.

How did you make sure your solution works?
Tested neo-boa contract detection in Visual DevTracker deploy menu before and after the change.

Are there any special changes in the code that we should be aware of?
Nope.

[meevee98]

Can you also update unit tests, please?

[ixje]

@EdgeDLT I understand the fix is for Visual DevTracker, but does this break deploying from neo-cli or have they updated it there as well? If so, will this mean that it will only work for e.g. neo-cli 2.11 or is it backwards compatible with 2.10.3? This question is mainly to understand if we should put a warning in the release notes. Thanks

As for the request from @meevee98 , she's referring to this

======================================================================
ERROR: test_abi_method_without_return_type (tests.test_abi_methods.TestContract)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/home/travis/build/CityOfZion/neo-boa/boa_test/tests/test_abi_methods.py", line 117, in test_abi_method_without_return_type
    self.assertEqual(main_function['returnType'], 'Void')
KeyError: 'returnType'
======================================================================
ERROR: test_abi_methods_only_entry_point (tests.test_abi_methods.TestContract)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/home/travis/build/CityOfZion/neo-boa/boa_test/tests/test_abi_methods.py", line 78, in test_abi_methods_only_entry_point
    self.assertEqual(main_function['returnType'], 'Any')
KeyError: 'returnType'
======================================================================
ERROR: test_abi_methods_success (tests.test_abi_methods.TestContract)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/home/travis/build/CityOfZion/neo-boa/boa_test/tests/test_abi_methods.py", line 22, in test_abi_methods_success
    self.assertEqual(main_function['returnType'], 'Any')
KeyError: 'returnType'
----------------------------------------------------------------------

[coveralls]

Coverage remained the same at 89.953% when pulling d238ffb on EdgeDLT:master into 7c7451c on CityOfZion:master.

[EdgeDLT]

@ixje as far as I'm aware this has been the standard format used by neo-cli for some time, nothing should have changed between 2.10.3 and 2.11.0. This output matches the NEP-3 standard.

[ixje]

I'll take it. Thanks!