include: remove extraneous const from uv_version()

Fixes the following warning:

  include/uv.h:236:30: warning: type qualifiers ignored on function
  return type [-Wignored-qualifiers]
   UV_EXTERN const unsigned int uv_version(void);
doc: update README, s/OS/PLATFORM/

Commit a9740c9 changed the name of the OS var to PLATFORM but forgot to
update the README.
build: simplify .buildstamp rule