Update http URLs to https (#152)

Use https URLs to improve pub score.
diff --git a/CHANGELOG.md b/CHANGELOG.md
index 7ea5e20..4f324d0 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,3 +1,5 @@
+## 1.6.1-dev
+
 ## 1.6.0
 
 * Remove `help` from the list of commands in usage.
diff --git a/README.md b/README.md
index b483c3f..e2d777b 100644
--- a/README.md
+++ b/README.md
@@ -383,8 +383,8 @@
       [ia32]      Intel x86
 ```
 
-[posix]: http://pubs.opengroup.org/onlinepubs/009695399/basedefs/xbd_chap12.html#tag_12_02
-[gnu]: http://www.gnu.org/prep/standards/standards.html#Command_002dLine-Interfaces
+[posix]: https://pubs.opengroup.org/onlinepubs/009695399/basedefs/xbd_chap12.html#tag_12_02
+[gnu]: https://www.gnu.org/prep/standards/standards.html#Command_002dLine-Interfaces
 [ArgParser]: https://pub.dev/documentation/args/latest/args/ArgParser/ArgParser.html
 [ArgParserException]: https://pub.dev/documentation/args/latest/args/ArgParserException-class.html
 [ArgResults]: https://pub.dev/documentation/args/latest/args/ArgResults-class.html
diff --git a/pubspec.yaml b/pubspec.yaml
index 47f5c40..68915fc 100644
--- a/pubspec.yaml
+++ b/pubspec.yaml
@@ -1,5 +1,5 @@
 name: args
-version: 1.6.0
+version: 1.6.1-dev
 homepage: https://github.com/dart-lang/args
 description: >-
  Library for defining parsers for parsing raw command-line arguments into a set