Yajl has not seen much activity upstream recently.
Switch to using Jansson >= 2.5.

All the platforms we target on https://libvirt.org/platforms.html
have a version >= 2.7 listed on the sites below:
https://repology.org/metapackage/jansson/versions
https://build.opensuse.org/package/show/devel:libraries:c_c++/libjansson

Additionally, Ubuntu 14.04 on Travis-CI has 2.5. Set the requirement
to 2.5 since we don't use anything from newer versions.

Implement virJSONValue{From,To}String using Jansson, delete the yajl
code (and the related virJSONParser structure) and report an error
if someone explicitly specifies --with-yajl.

Also adjust the test data to account for Jansson's different whitespace
usage for empty arrays and tune up the specfile to keep 'make rpm'
working when bisecting.
Signed-off-by: NJán Tomko <jtomko@redhat.com>