Take new rapidjson
Merge pull request #761 from Metaswitch/rapidjson

[DON'T MERGE] [REVIEWER: AMC] Take new rapidjson
exclude rapidjson from coverage reports
Merge pull request #764 from Metaswitch/rapidjson_coverage

No reviewer: exclude rapidjson from coverage reports
Change Metaswitch submodule refs to ClearwaterCore