travis: test on the oldest supported SDK (#34)

diff --git a/.travis.yml b/.travis.yml
index 44a0542..18267fc 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -2,7 +2,7 @@
 
 dart:
   - dev
-  - stable
+  - 2.0.0
 
 dart_task:
   - test: --platform vm,chrome
diff --git a/pubspec.yaml b/pubspec.yaml
index 0cb9326..813dd3b 100644
--- a/pubspec.yaml
+++ b/pubspec.yaml
@@ -1,12 +1,12 @@
 name: source_span
-version: 1.5.5
+version: 1.5.6-dev
 
 description: A library for identifying source spans and locations.
 author: Dart Team <misc@dartlang.org>
 homepage: https://github.com/dart-lang/source_span
 
 environment:
-  sdk: '>=1.8.0 <3.0.0'
+  sdk: '>=2.0.0 <3.0.0'
 
 dependencies:
   charcode: ^1.0.0
@@ -14,4 +14,4 @@
   term_glyph: ^1.0.0
 
 dev_dependencies:
-  test: '>=0.12.0 <2.0.0'
+  test: ^1.0.0