configure.ac: Bump libsoup requirement as libsoup does not support async client opera...

configure.ac: Bump libsoup requirement as libsoup does not support async client operation prior to version 2.2.104 an...

Original commit message from CVS:
Patch by: Wouter Cloetens <wouter at mind dot be>
* configure.ac:
Bump libsoup requirement as libsoup does not support async client
operation prior to version 2.2.104 and it has some leaks.
* ext/soup/gstsouphttpsrc.c: (gst_souphttp_src_class_init),
(gst_souphttp_src_init), (gst_souphttp_src_dispose),
(gst_souphttp_src_set_property), (gst_souphttp_src_create),
(gst_souphttp_src_start), (gst_souphttp_src_stop),
(gst_souphttp_src_unlock), (gst_souphttp_src_unlock_stop),
(gst_souphttp_src_get_size), (soup_got_headers), (soup_got_body),
(soup_finished), (soup_got_chunk), (soup_response),
(soup_session_close):
* ext/soup/gstsouphttpsrc.h:
Implement unlock().
Picks up the size from the Content-Length header and emit a duration
message.
Don't leak the GMainContext object.
Fixes #500099.