Fix python bindings installation in case of empty DESTDIR

Fix python bindings installation in case of empty DESTDIR

The current behaviour is to install the extension to ./usr/local (if
prefix is /usr/local), so it ends up in the temporary build directory
instead of the intended absolute prefix.

The slash after $(DESTDIR) is needed to make empty DESTDIR mean
installation to / instead of making the prefix to mean a relative
path. We make it triple slash as POSIX reserves double slashes to
possibly have a special treatment by the OS.

The change for travis is needed as "make install" is run as root, and
once the python extension is actually installed, it leaves a
root-owned .egg-info file.