.travis.yml: install protobuf-c into /usr

see if this fixes the build error:

make  check-TESTS
make[3]: Entering directory `/home/travis/build/protobuf-c/protobuf-c-rpc/protobuf-c-rpc-0.1/_build'
/home/travis/build/protobuf-c/protobuf-c-rpc/protobuf-c-rpc-0.1/_build/t/.libs/lt-test-rpc: error while loading shared libraries: libprotobuf-c.so.1: cannot open shared object file: No such file or directory