Update the bundled hiredis code to version 0.11.0.

[jparise]

This fixes clang 4.1-based builds.

hiredis.c:696:31: error: second argument to 'va_arg' is of incomplete type 'void'
                    va_arg(ap,void);
                    ~~~~~~~~~~^~~~~
/usr/bin/../lib/clang/4.1/include/stdarg.h:35:50: note: expanded from macro 'va_arg'

Alternatively, hiredis could be included as a git submodule (as is done in hiredis-py).

[jparise]

@antirez, I'm interested in your opinion on the best approach here. Do you prefer the way I upgraded the hiredis library in-line, or should this redis-tools include hiredis as a git submodule?