Allow vm_service >=1.0.0 <3.0.0  (#274)

This loosens the vm_service version constraint to include 2.x releases.
package:test, in particular, now applies a ^2.0.0 constraint to vm_service,
which had made it incompatible with this package.
diff --git a/pubspec.yaml b/pubspec.yaml
index e4e9a35..d701210 100644
--- a/pubspec.yaml
+++ b/pubspec.yaml
@@ -13,7 +13,7 @@
   package_config: '>=0.1.5 <2.0.0'
   path: '>=0.9.0 <2.0.0'
   stack_trace: ^1.3.0
-  vm_service: ^1.0.0
+  vm_service: '>=1.0.0 <3.0.0'
 
 dev_dependencies:
   test: ^1.0.0