Use new "keep" option instead of "strip" option

[james-johnston-thumbtack]

It looks like the "strip" option used by bimg here:

bimg/vips.h

Lines 327 to 333 in ee92010

	return vips_jpegsave_buffer(in, buf, len,
	"strip", INT_TO_GBOOLEAN(strip),
	"Q", quality,
	"optimize_coding", TRUE,
	"interlace", INT_TO_GBOOLEAN(interlace),
	NULL
	);

Was recently deprecated and replaced with a new "keep" option: libvips/libvips@32ddfe7#diff-0cdb749283812170ac7d97de48ef99c912116600da659cc717b249eecc350151

It looks like the old "strip" flag is still supported.... for now:
https://github.com/libvips/libvips/blob/4cb9da6248e8134050aea7c4adc5d696f5d399d9/libvips/foreign/foreign.c#L1789-L1795

It seems that bimg could consider supporting the new "keep" option for libvips versions that support it.