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

A missing " prevented the DPDK build in the matrix from functioning.
Correctly build DPDK as a single, shared library and point the OVS
build to the corresponding build directory. Also removes the 'make
install' as it is not needed.

Due to incorrect casts in the DPDK headers, we have to disable
bad-function-cast 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>