update min SDK to 2.7 (#32)

.travis.yml

diff --git a/.travis.yml b/.travis.yml
index 51308e5..59d861d 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -1,7 +1,7 @@
 language: dart
 
 dart:
-  - 2.3.0
+  - 2.7.0
   - dev
 
 dart_task:

CHANGELOG.md

diff --git a/CHANGELOG.md b/CHANGELOG.md
index 6b34efd..b9ad843 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,3 +1,7 @@
+## 3.1.5-dev
+
+* Require minimum Dart SDK of `2.7.0`.
+
 ## 3.1.4
 
 * Fixed lints affecting package health score.

pubspec.yaml

diff --git a/pubspec.yaml b/pubspec.yaml
index b3df0c4..eccff2c 100644
--- a/pubspec.yaml
+++ b/pubspec.yaml
@@ -1,11 +1,11 @@
 name: http_parser
-version: 3.1.4
+version: 3.1.5-dev
 description: >-
   A platform-independent package for parsing and serializing HTTP formats.
 homepage: https://github.com/dart-lang/http_parser
 
 environment:
-  sdk: '>=2.3.0 <3.0.0'
+  sdk: '>=2.7.0 <3.0.0'
 
 dependencies:
   charcode: ^1.1.0