stdlib.json.decoder: Hot fix for json decoder

[corona10]

a = (1,)

c, = a
print(type(c))
# should be <type 'int'>
# but <type 'tuple'> on grumpy

There is something wrong behavior on the tuple assignment of grumpy.
This PR does not fix the essential problem but fix to use json library.

Updates: #3

[corona10]

I think that we need to integrate Travis CI also.
WDYT? @alanjds

[alanjds]

Travis is now aware of it 👍

[alanjds]

Will deal with Gitlab later too.

Gitlab builds on a more vanilla Docker, and eases getting issues about the Travis environment when it happens.