Make an int64->double conversion explicit to silence -Wconversion.

Addresses issue #173:
  https://code.google.com/p/googlemock/issues/detail?id=173

src/gtest.cc

@@ -3501,7 +3501,7 @@ std::string XmlUnitTestResultPrinter::RemoveInvalidXmlCharacters(
 // Formats the given time in milliseconds as seconds.
 std::string FormatTimeInMillisAsSeconds(TimeInMillis ms) {
   ::std::stringstream ss;
-  ss << ms/1000.0;
+  ss << (static_cast<double>(ms) * 1e-3);
   return ss.str();
 }