protobuf-c: eliminate void pointer arithmetic

gcc silently treats arithmetic on a void pointer as char pointer
arithmetic without -pedantic. Other compilers do not. Make it explicit
that we are doing arithmetic on a pointer with object size 1.

This eliminates the following diagnostic:

    protobuf-c/protobuf-c.c: In function 'parse_packed_repeated_member':
    protobuf-c/protobuf-c.c:2404:34: warning: pointer of type 'void *' used in arithmetic [-Wpedantic]
      void *array = *(void **) member + siz * (*p_n);

(I did not find any other instances of 'void *' arithmetic when
compiling with -pedantic.)

Based on a patch from alex85k (#167).
.travis.yml: bump protobuf version to 2.6.0

Build with the latest protobuf release, in preparation for adding
"oneof" support.
.travis.yml: fetch the protobuf tarball from GitHub instead of Google Code

Per the protobuf developers,

    We will from everything away from Google Code eventually. We haven't
    decided where to put future release packages yet but as it seems github
    supports this well chances are we'll use github as the canonical location
    for all downloads.