travis: Scan patch codestyle

Checkpatch is a tool which checks patch code style
alignment with Linux kernel code.
By the way print short graphs of used commits, to
show whats under test.
Checkpatch call is precede by `set -x`, to show used
arguments values, especially checked commits range.

Signed-off-by: Karol Trzcinski <karolx.trzcinski@linux.intel.com>