add built collection as official dep

Author: davidmarne

pubspec.yaml

@@ -8,6 +8,7 @@ homepage: https://github.com/davidmarne/built_redux
 dependencies:
   analyzer: '>=0.29.0 <0.31.0'
   build: ^0.10.0
+  built_collection: ^1.0.0
   built_value: ^4.1.0
   source_gen: '^0.7.0'
 
@@ -18,7 +19,6 @@ dev_dependencies:
   dart_style: '>=0.2.4 <2.0.0'
   coverage: ^0.7.3
   test: "^0.12.0"
-  built_collection: "^1.0.0"
 
 environment:
   sdk: ">=1.17.1 <2.0.0"