trex: Update T-Rex to v2.38

trex: Update T-Rex to v2.38

Default API version of T-Rex was changed to v2.38. T-Rex related
documentation was updated to inform user, that the same version
of T-Rex must be used for both server and client.

JIRA: VSPERF-564

Change-Id: Ib1bedde683efafa84a6069b55daa0507700dbb30
Signed-off-by: Martin Klozik <martinx.klozik@intel.com>
Reviewed-by: Al Morton <acmorton@att.com>
Reviewed-by: Christian Trautman <ctrautma@redhat.com>
Reviewed-by: Sridhar Rao <sridhar.rao@spirent.com>
Reviewed-by: Richard Elias <richardx.elias@intel.com>