Widen constraint on package_config

package_config has released version 1.0.0, which is compatible with earlier versions.  Relax constraint to <2.0.0.   This could also be changed to ^1.0.0 if you want.

Author: whesse

pubspec.yaml

@@ -15,7 +15,7 @@ dependencies:
   logging: '>=0.9.0 <0.12.0'
   markdown: ^0.10.1
   mustache4dart: ^1.0.9
-  package_config: ^0.1.5
+  package_config: '>=0.1.5 <2.0.0'
   path: ^1.3.0
   quiver: '>=0.18.0 <0.23.0'
   resource: ^1.1.0