Loosen built_value lower bound

+ Turns out some consumers had not yet got off of 4.x (sadpanda)

Author: aaronlademann-wf

pubspec.yaml

@@ -11,7 +11,7 @@ dependencies:
   analyzer: ">=0.30.0+4 <=0.31.0"
   barback: ">=0.15.2 <=0.15.2+14"
   built_redux: ^7.4.2
-  built_value: ">=5.1.3 <5.2.0" # >=5.2.0 is Dart 2 SDK only
+  built_value: ">=4.6.1 <5.2.0" # >=5.2.0 is Dart 2 SDK only
   js: ^0.6.1+1
   logging: ">=0.11.3+2 <1.0.0"
   meta: ^1.1.6