Switch badges in README to master branch
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ddemidov/amgcl,dondieselkopf/amgcl,ddemidov/amgcl,huahbo/amgcl,huahbo/amgcl,ddemidov/amgcl,ddemidov/amgcl,huahbo/amgcl,dondieselkopf/amgcl,dondieselkopf/amgcl