travis: use gcc-5 instead of gcc(-4.9)

The std::put_time and std::get_time introduced in
9febc2b238e1e322dce1f94ecbed46d595893b52 are part of C++11, but not
implemented in GCC until version 5. std::put_time could actually be
worked around via using the facets put() directly, but get() isn't
implemented so that doesn't really help.

We require various tools from wily (which also means we can't build apt
on Debian stable) already, so requiring gcc-5 is just one more instead
of a big step [and an ignoreable change for changelog anyhow].

It also helps in testing what will actually be used (in terms of the
c++11 std ABI) instead of the old ABI.

Git-Dch: Ignore

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