fix isPromotable ring family

This reverts commit 63d7fc7.

Author: pzinn

M2/Macaulay2/m2/reals.m2

@@ -106,10 +106,13 @@ promote(RawRingElement,RingElement) := (x,R) -> new R from x
 promote(Number,InexactNumber) := (x,RR) -> promote(x,default RR)
 promote(ZZ,RR') := 
 promote(QQ,RR') := 
+promote(RR',RR') :=
 promote(RR,RR') := (i,K) -> toRR(K.precision,i)
 promote(ZZ,CC') := 
 promote(QQ,CC') := 
 promote(RR,CC') := 
+promote(RR',CC') :=
+promote(CC',CC') :=
 promote(CC,CC') := (i,K) -> toCC(K.precision,i)
 promote(ZZ,RRi') :=
 promote(QQ,RRi') :=