travis: Enable -Wsparse-error and fail on new sparse warnings

A new option -Wsparse-error has been introduced to sparse in the
commits:

	4d8811 sparse: Make -Werror turn warnigns into errors
	fe57af rename -Werror to -Wsparse-error

which makes sparse fail on warnings. Start using the sparse git tree
for verification and make use of the new flag.

Signed-off-by: Thomas Graf <tgraf@noironetworks.com>
Signed-off-by: Ben Pfaff <blp@nicira.com>