Better feature detection for assimp version

The unified headers were introduced in Assimp 2.0.1150, so checking for Assimp 3.0.0 is not quite the best solution.

See https://github.com/assimp/assimp/commit/6fa251c2f2e7a142bb861227dce0c26362927fbc
Invert Assimp version detect logic for greater accuracy
Remove stray IS_ASSIMP3 define
Merge pull request #14 from cottsay/hydro-devel

Better feature detection for assimp version