Fixed __query_virustotal return type

[DocArmoryTech]

__query_virustotal returned a Response object and not the json expected; modified so that report_json is returned instead of report.

[coveralls]

Coverage remained the same at 38.024% when pulling 4ee4db1 on DocArmoryTech:patch-1 into 12e05fd on MISP:master.

[codecov]

Codecov Report

Merging #562 into master will not change coverage by %.
The diff coverage is 0.00%.

@@           Coverage Diff           @@
##           master     #562   +/-   ##
=======================================
  Coverage   38.07%   38.07%           
=======================================
  Files          35       35           
  Lines        7068     7068           
=======================================
  Hits         2691     2691           
  Misses       4377     4377           

Impacted Files	Coverage Δ
pymisp/tools/vtreportobject.py	29.78% <0.00%> (ø)

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[Rafiot]

Oh. Your're right, thanks!