Update v0.6.1 what's new link and date

- remove ubuntu installation commands from Readme, but users can still find it on the donwload page.

README.md

@@ -8,9 +8,9 @@
 
 ## Download ###
 
-### Pencil2D 0.6.1  (15 April 2018)
+### Pencil2D 0.6.1  (16 April 2018)
 
-[What's new?](https://www.pencil2d.org/2017/12/introducing-pencil2d-0.6.html)
+[What's new?](https://www.pencil2d.org/2018/04/maintenance-release-0.6.1.html)
 
 | Windows 64 bit   | Windows 32 bit    | Mac             | Linux             |
 | :--------------: | :---------------: | :-------------: | :---------------: |
@@ -21,18 +21,6 @@
 [2]: https://github.com/pencil2d/pencil/releases/download/v0.6.1.1/pencil2d-mac-0.6.1.1.zip
 [3]: https://github.com/pencil2d/pencil/releases/download/v0.6.1.1/pencil2d-linux-amd64-0.6.1.1.AppImage
 
-### Debian & Ubuntu
-
-```bash    
-sudo apt-get install pencil2d
-```
-
-### Homebrew Cask (macOS)
-
-```bash
-brew cask install pencil2d
-```
-
 ### Nightly build
 
 Nightly builds are the bleeding edge versions of Pencil2D, which contains the most recent fixes and features.