Update README and configure.ac message concerning double-conversion

Documentation about double-conversion seems outdated.

* #include path has changed
* SConstruct.double-conversion is no longer present, but was not necessary at all for successful `make`/`make check` for me.

As to the second point, the file was added years ago and removed last month.
I tried with latest release (v.1.1.5) and current master and `make check` just worked with both.