Drop support for older Barback versions.

lib/src/barback.dart

Index: lib/src/barback.dart
diff --git a/lib/src/barback.dart b/lib/src/barback.dart
index 33c775425b1e7f466570c8b83bed6e7d549c16a4..fcc4c5f8ea71e6b6a0358da3e0f8f5fe9d20b713 100644
--- a/lib/src/barback.dart
+++ b/lib/src/barback.dart
@@ -37,7 +37,7 @@ import 'package:pub_semver/pub_semver.dart';
 ///
 /// [compat]: https://gist.github.com/nex3/10942218
 final pubConstraints = {
-  "barback": new VersionConstraint.parse(">=0.13.0 <0.15.3"),
+  "barback": new VersionConstraint.parse(">=0.15.0 <0.15.3"),
   "source_span": new VersionConstraint.parse(">=1.0.0 <2.0.0"),
   "stack_trace": new VersionConstraint.parse(">=0.9.1 <2.0.0")
 };