Remove plugins before copying them again.

This way plugins can rename/delete files w/o having old files lying around.
Only delete core plugins.
Use PLUGIN_PATH in dokku default case.
port 'wait_to_retire' from broadly/dokku
Actually suppress the output from the git commands
Update plugins.md
Merge pull request #717 from kristofsajdak/patch-1

Update plugins.md
Update vagrant installation instructions

Avoids 
```
Permission denied (publickey).
fatal: The remote end hung up unexpectedly
```
that can occur on certain Vagrant images. Since we need readonly anyway, might as well use HTTPS instead of SSH.
Fix `plugins-install-dependencies` [fixes #720]
Merge pull request #721 from wrboyce/master

Fix `plugins-install-dependencies` [fixes #720]
Let the user know it's on the `stack` target

Since curl is running quietly it may give the impression that the installation hanged with the last message on the terminal being: "sleep 2 # give docker a moment i guess". In my case I thought it gave it a huge moment and not 2 seconds. It wasn't until the third time that I actually checked the processes on the machine and saw there it was curl downloading stuff quietly.
Merge pull request #723 from ghostbar/patch-1

Let the user know it's on the `stack` target
Fixing tabs issue

Sorry
Merge pull request #724 from ghostbar/patch-2

Fixing tabs issue
Merge pull request #664 from alexernst/fix/hostname-url

Don't overwrite $APP/URL modified by plugins in post-deploy hook
deis sponsorship
Redirect to the app deployment docs on success.
Merge pull request #735 from andrewsomething/master

Redirect to the app deployment docs on success.
Update README.md

Sorry, that typo was getting to me :)
Merge pull request #745 from rcarmo/patch-1

Update README.md