ShufflePlug : Fix build breakage
ShufflePlug : Fix build breakage

We merged ShufflePlug to 0.56_maintenance where `Context::Substitutions` still existed, but then merged it forwards to master where it has been replaced by `IECore::StringAlgo::Substitutions`.

johnhaddon/gaffer,boberfly/gaffer,ImageEngine/gaffer,boberfly/gaffer,andrewkaufman/gaffer,lucienfostier/gaffer,hradec/gaffer,hradec/gaffer,johnhaddon/gaffer,lucienfostier/gaffer,andrewkaufman/gaffer,GafferHQ/gaffer,lucienfostier/gaffer,GafferHQ/gaffer,johnhaddon/gaffer,ImageEngine/gaffer,andrewkaufman/gaffer,johnhaddon/gaffer,johnhaddon/gaffer,boberfly/gaffer,GafferHQ/gaffer,lucienfostier/gaffer,GafferHQ/gaffer,ImageEngine/gaffer,hradec/gaffer,andrewkaufman/gaffer,hradec/gaffer,GafferHQ/gaffer,andrewkaufman/gaffer,hradec/gaffer,boberfly/gaffer,ImageEngine/gaffer