Update build system and Travis integration

[fingolfin]

Travis:

• test against GAP 4.9, 4.10, 4.11 and master
• install zlib headers so that GAP doesn't have to compile zlib

Build system:

• put common GAP kernel extension build rules into Makefile.gappkg

[fingolfin]

There is actually an error in the code which makes the compiler file with this message:

src/GaloisGroups.c:12:36: error: ‘i’ undeclared (first use in this function)
     pari_APPLY_same(FpXQ_pow(gel(x,i),p,T,p));

Indeed, looking into that file, I see this function (added by @videlec in commit 89dc33c):

static GEN
FpXQV_pow(GEN x, GEN T, GEN p)
{
    pari_APPLY_same(FpXQ_pow(gel(x,i),p,T,p));
}

That i is not defined, it seems?

[fingolfin]

@videlec @markuspf do you have any hints for me how FpXQV_pow really should look like?