Travis : Add debug builds to matrix.
Travis : Add debug builds to matrix.

Fixes #1781.

ImageEngine/gaffer,ImageEngine/gaffer,chippey/gaffer,ivanimanishi/gaffer,andrewkaufman/gaffer,johnhaddon/gaffer,andrewkaufman/gaffer,andrewkaufman/gaffer,appleseedhq/gaffer,hradec/gaffer,GafferHQ/gaffer,andrewkaufman/gaffer,lucienfostier/gaffer,boberfly/gaffer,GafferHQ/gaffer,ImageEngine/gaffer,johnhaddon/gaffer,ImageEngine/gaffer,appleseedhq/gaffer,hradec/gaffer,ivanimanishi/gaffer,ivanimanishi/gaffer,johnhaddon/gaffer,lucienfostier/gaffer,appleseedhq/gaffer,lucienfostier/gaffer,GafferHQ/gaffer,appleseedhq/gaffer,johnhaddon/gaffer,lucienfostier/gaffer,chippey/gaffer,boberfly/gaffer,GafferHQ/gaffer,andrewkaufman/gaffer,boberfly/gaffer,ivanimanishi/gaffer,boberfly/gaffer,GafferHQ/gaffer,hradec/gaffer,hradec/gaffer,chippey/gaffer,hradec/gaffer,chippey/gaffer,johnhaddon/gaffer