Minor fixes; update docs

• Functional changes
  • Serialize UTF-8 string with Unicode escapes (#687)
    • This will break anyone who relied on not Unicode-escaping their encoding strings.
    • See discussion at #687. (A future release might allow skipping all Unicode escape.)
  • Un-deprecate removeMember overloads, return void (#693)
  • Allow Json::Value to be used in a boolean context (#695)
  • Use move ctor in append() (#677)
  • Add value_type to improve integration with boost (#671)
• Meta-changes
  • Fix meson.build to allow using jsoncpp as a subproject
  • Fix some compiler warnings
  • Update docs
  • Update for TravisCI