title: error: cannot pass object of non-trivial type 'std::string' and more errors
body: <p>I'm running a build in Travis CI, and during the compilation, this error occurs. I tried specifying the C++ compiler and tried using g++ but that resulted in more errors.</p>
  
  <pre class="lang-sh prettyprint-override"><code>$ clang -I ./jsoncpp/include/ -L ./jsoncpp/build/debug/lib -std=c++14 -v test.cpp -o buildtest.exe
  ...
  ./IBMWatson.h:79:44: error: cannot pass object of non-trivial type 'std::string'
        (aka 'basic_string&lt;char&gt;') through variadic function; call will abort at
        runtime [-Wnon-pod-varargs]
                  curl_easy_setopt(curl, CURLOPT_PASSWORD, apikey); /* Par...
                                                           ^
  ./IBMWatson.h:81:39: error: cannot pass object of non-trivial type 'std::string'
        (aka 'basic_string&lt;char&gt;') through variadic function; call will abort at
        runtime [-Wnon-pod-varargs]
                  curl_easy_setopt(curl, CURLOPT_URL, url); /* Sets Regio...
                                                      ^
  ./IBMWatson.h:104:102: warning: result of comparison against a string literal is
        unspecified (use strncmp instead) [-Wstring-compare]
    ...+ response.length(), &amp;root, &amp;err) || funcName == "returnVoices" || funcN...
  ...
  4 warnings and 4 errors generated.
  The command "clang -I ./jsoncpp/include/ -L ./jsoncpp/build/debug/lib -std=c++14 -v test.cpp -o buildtest.exe" exited with 1.
  </code></pre>