requirements: update elfi requirement from <0.8.5 to <0.8.7 (#2608)

requirements.txt

@@ -1,7 +1,7 @@
 autograd<1.6
 graph-scheduler>=0.2.0, <1.1.2
 dill<=0.32
-elfi<0.8.5
+elfi<0.8.7
 graphviz<0.21.0
 grpcio<1.43.0
 grpcio-tools<1.43.0