Trade: bump the AbstractImporter plugin interface string.

This is one of those nasty ABI incompatibilites that shuffles functions
around, so better force a recompile this way.

src/Magnum/Trade/AbstractImporter.cpp

@@ -62,7 +62,7 @@ namespace Magnum { namespace Trade {
 std::string AbstractImporter::pluginInterface() {
     return
 /* [interface] */
-"cz.mosra.magnum.Trade.AbstractImporter/0.3.3"
+"cz.mosra.magnum.Trade.AbstractImporter/0.3.4"
 /* [interface] */
     ;
 }