travis: Update sparse git repo

travis: Update sparse git repo

The old git tree git://git.kernel.org/pub/scm/devel/sparse/chrisl/sparse.git
has not been updated since 2016, and that triggers the following build error
on Ubuntu 18.04 host with 2.27-3 libc6-dev.  So update the sparse git repo
to the new one.

$ .travis/linux-prepare.sh
$  export PATH=$PATH:$HOME/bin
$ .travis/linux-build.sh

/usr/include/stdlib.h:140:17: error: Expected ; at end of declaration
/usr/include/stdlib.h:140:17: error: got strtof32
/usr/include/stdlib.h:146:17: error: Expected ; at end of declaration
/usr/include/stdlib.h:146:17: error: got strtof64
/usr/include/stdlib.h:158:18: error: Expected ; at end of declaration
/usr/include/stdlib.h:158:18: error: got strtof32x
/usr/include/stdlib.h:233:33: error: Expected ) in function declarator
/usr/include/stdlib.h:233:33: error: got __f
/usr/include/stdlib.h:239:33: error: Expected ) in function declarator
/usr/include/stdlib.h:239:33: error: got __f
/usr/include/stdlib.h:251:35: error: Expected ) in function declarator
/usr/include/stdlib.h:251:35: error: got __f
/usr/include/stdlib.h:316:17: error: Expected ; at end of declaration
/usr/include/stdlib.h:316:17: error: got strtof32_l
/usr/include/stdlib.h:323:17: error: Expected ; at end of declaration
/usr/include/stdlib.h:323:17: error: got strtof64_l
/usr/include/stdlib.h:337:18: error: Expected ; at end of declaration
/usr/include/stdlib.h:337:18: error: got strtof32x_l
Makefile:5288: recipe for target 'lib/aes128.lo' failed
make[2]: *** [lib/aes128.lo] Error 1
...

Tested on Jarvis: https://travis-ci.org/YiHungWei/ovs/builds/521979625

Signed-off-by: Yi-Hung Wei <yihung.wei@gmail.com>
Signed-off-by: Ben Pfaff <blp@ovn.org>