travis: Run 'make distcheck' instead of 'make check'

make distcheck runs a superset of make check and will additionally
catch failures in adding new files to the Makefile. It will also test
installation and uninstallation of the package.

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