libsdformat4 debian should declare it depends on ignition-math2 > 2.6.0

**Original report ([archived issue](https://osrf-migration.github.io/sdformat-gh-pages/#!/osrf/sdformat/issues/179)) by Shane Loretz (Bitbucket: [Shane Loretz](https://bitbucket.org/%7B656e3311-aad9-45a1-aaf7-b0ee0e84b287%7D/), GitHub: [sloretz](https://github.com/sloretz)).**

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See https://bitbucket.org/osrf/gazebo/issues/2448/problem-running-gazebo7

Users are getting a newer version of libsdformat that [depends on a class added in 2.6.0](https://bitbucket.org/osrf/gazebo/issues/2448/problem-running-gazebo7#comment-44458354).

Apt reports the dependencies as
`Depends: sdformat-sdf (>= 4.0.0-1ubuntu2), libboost-filesystem1.58.0, libboost-regex1.58.0, libboost-system1.58.0, libc6 (>= 2.14), libgcc1 (>= 1:3.0), libignition-math2, libstdc++6 (>= 5.2), libtinyxml2.6.2v5, liburdfdom-model0.4`

Maybe `libignition-math2` should have `(>= 2.6.0)` added to it. Where is that information kept? `osrf/release-tools`?