travis: Fix DPDK build and treat bad-function-cast warning as non-error

travis: Fix DPDK build and treat bad-function-cast warning as non-error

A missing " prevented the DPDK build in the matrix from functioning
so far. This patch enables the DPDK build by properly building DPDK
as a single library and by pointing the OVS build to the corresponding
build directory. Also removes the 'make install' as it is not required
and only slows down the build.

Due to incorrect casts in the DPDK headers, we have to disable
bad-function-cast and cast-align warnings as being treated as errors
for now.

Signed-off-by: Thomas Graf <tgraf@noironetworks.com>
Co-authored-by: Daniele Di Proietto <ddiproietto@vmware.com>
Signed-off-by: Ben Pfaff <blp@nicira.com>