travis: Run unit tests on i386 arch
travis: Run unit tests on i386 arch

To do more tests for the compatibility with the 32-bit architecture,
this patch fixes to run "go build" in _unittest section and run unit
tests with GOARCH="386".

Signed-off-by: IWASE Yusuke <bb80e84cc9f5a8f01385872ee89032beb5d4eba7@gmail.com>

a16/gobgp,fujita/gobgp,osrg/gobgp,osrg/gobgp,a16/gobgp,a16/gobgp,osrg/gobgp,j-kato/gobgp,tamihiro/gobgp,tamihiro/gobgp,a16/gobgp,osrg/gobgp,a16/gobgp,fujita/gobgp,a16/gobgp,j-kato/gobgp,a16/gobgp,tamihiro/gobgp,j-kato/gobgp,tamihiro/gobgp,fujita/gobgp,fujita/gobgp