[Tile-serving] [osm2pgsql] Fix warnings when compiling tests, use -Werror with travis (#395)

[Paul Norman]

Travis is passing with Clang, but g++ is turning up an error in the zlib detection autoconf script, so this still needs more work

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https://github.com/openstreetmap/osm2pgsql/pull/395#issuecomment-121782891
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