make docker build_binaries.sh macOS compatible

[frederikvs]

On my macbook build_binaries.sh didn't work :

readlink: illegal option -- f
usage: readlink [-n] [file ...]

Turns out that readlink is something slightly different here :

STAT(1)                   BSD General Commands Manual                  STAT(1)

NAME
     readlink, stat -- display file status

[...]

     -f format
             Display information using the specified format.  See the FORMATS
             section for a description of valid formats.

However, regarding the -f flag that is used, the readlink man page for linux also mentions

realpath(1) is the preferred command to use for canonicalization functionality.

And that command also works on macOS :-)

[guillaumelecerf]

Maybe you should change .travis.yml accordingly for consistency.

[frederikvs]

good point @guillaumelecerf , I wasn't aware it was being used in different places. .travis.yml has also been updated, and a quick grep shows that there are no other places where readlink -f was being used.

[kanflo]

Thanks!