Compile error with g++-4.8

[springmeyer]

Seeing that clang++ is working with master branch but not g++, on travis (https://travis-ci.org/osmcode/libosmium/jobs/18206306#L581).

g++-4.8 -O3 -std=c++11 -D_LARGEFILE_SOURCE -D_FILE_OFFSET_BITS=64 -I../include -Wall -Wextra -pedantic -Wredundant-decls -Wdisabled-optimization -Wctor-dtor-privacy -Wnon-virtual-dtor -Woverloaded-virtual -Wsign-promo -Wold-style-cast -o osmium_mpdump osmium_mpdump.cpp  -lexpat -pthread -lz -lprotobuf-lite -losmpbf -lz -lbz2
In file included from ../include/osmium/relations/collector.hpp:51:0,
                 from ../include/osmium/area/collector.hpp:44,
                 from osmium_mpdump.cpp:17:
../include/osmium/relations/detail/member_meta.hpp: In function ‘constexpr bool osmium::relations::operator<(const osmium::relations::MemberMeta&, const osmium::relations::MemberMeta&)’:
../include/osmium/relations/detail/member_meta.hpp:102:32: error: call to non-constexpr function ‘osmium::object_id_type osmium::relations::MemberMeta::member_id() const’