Fix: vppapigen make build fails on fresh install

Steps to reproduce:
  vagrant@localhost:/vagrant$ build-root/vagrant/build.sh
  ...
  @@@@ Building vpp in /vagrant/build-root/build-vpp-native/vpp @@@@
  [51/1169] Generating API header /vagrant/build-root/build-vpp-native/vpp/vlibmemory/memclnt.api.json
  FAILED: cd /vagrant/build-root/build-vpp-native/vpp/vlibmemory && mkdir -p /vagrant/build-root/build-vpp-native/vpp/vlibmemory && /vagrant/src/tools/vppapigen/vppapigen --includedir /vagrant/src --input /vagrant/src/vlibmemory/memclnt.api JSON --output /vagrant/build-root/build-vpp-native/vpp/vlibmemory/memclnt.api.json
  AttributeError: 'module' object has no attribute 'dumps'

  This seems to be due to JSON.py namespace colliding with the standard lib json.py

Change-Id: If389e4e05ef0c166b0c2b3bef7ec0185298679a8
Signed-off-by: Paul Vinciguerra <pvinci@vinciconsulting.com>