Compile static luajit library with -fPIC

Should fix the following travis error:

    /usr/bin/ld: /opt/neovim-deps/lib/libluajit-5.1.a(lj_err.o): relocation R_X86_64_32S against `.rodata' can not be used when making a shared object; recompile with -fPIC
    /opt/neovim-deps/lib/libluajit-5.1.a: could not read symbols: Bad value