Bump version to release 1.2.8. (#823)

* Bump version to release 1.2.8.

* Roll Travis to Dart 2.3.0.

dart_style uses the UI as code stuff now, so it doesn't run on older
SDKs.

.travis.yml

diff --git a/.travis.yml b/.travis.yml
index 2477385..93d3428 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -1,7 +1,7 @@
 language: dart
 
 dart:
-  - 2.1.0
+  - 2.3.0
   - dev
 
 dart_task:

bin/format.dart

diff --git a/bin/format.dart b/bin/format.dart
index 5a70dc4..98e4d95 100644
--- a/bin/format.dart
+++ b/bin/format.dart
@@ -15,7 +15,7 @@
 import 'package:dart_style/src/style_fix.dart';
 
 // Note: The following line of code is modified by tool/grind.dart.
-const version = "1.2.7";
+const version = "1.2.8";
 
 void main(List<String> args) {
   var parser = ArgParser(allowTrailingOptions: true);

pubspec.yaml

diff --git a/pubspec.yaml b/pubspec.yaml
index 1cfc78c..3f2b24c 100644
--- a/pubspec.yaml
+++ b/pubspec.yaml
@@ -1,6 +1,6 @@
 name: dart_style
 # Note: See tool/grind.dart for how to bump the version.
-version: 1.2.8-dev
+version: 1.2.8
 author: Dart Team <misc@dartlang.org>
 description: >-
   Opinionated, automatic Dart source code formatter.