DSL: add `conflicts_with` stanza

Forgotten in #4688, but should be considered part of Cask DSL 1.0.
A `depends_on` stanza is much less useful without the corresponding
`conflicts_with`.

References: #4896
Add Firewall Builder version 5.1.0.3599
Merge pull request #5569 from rolandwalker/dsl_conflicts_with

DSL: add `conflicts_with` stanza
Corrected url to download latest version of VirtualHostX.
SourceTree 1.9.7
Merge pull request #5577 from ahbeng/sourcetree-1.9.7

SourceTree 1.9.7
bug: use install_phase in caskroom_only / qlplugin

Missed in #4865.  install method was renamed.
Bug exercised by Cask amazon_music.rb.
updated googleappenginelauncher to 1.9.7
Add formula for pngyu.
Merge pull request #5579 from rolandwalker/caskroom_only_install_phase

bug: use `install_phase` in `caskroom_only` and `qlplugin`
Merge pull request #5580 from rsenkbeil/pngyu

Add formula for pngyu.
Merge pull request #5578 from earlonrails/master

updated googleappenginelauncher to 1.9.7
gitx: Create symlink to gitx executable
Merge pull request #5581 from sjackman/gitx

gitx: Create symlink to gitx executable
Merge pull request #5576 from lgw4/virtualhostx

Corrected url to download latest version of VirtualHostX.
new kindlegen sha256 and directory layout

closes #5574
Update LibreOffice.app to v4.3.0
Merge pull request #5583 from jconley/libreoffice

Upgrade LibreOffice.app to v4.3.0
Merge pull request #5575 from alexguedes/firewall-builder

Add Firewall Builder version 5.1.0.3599
Merge pull request #5582 from rolandwalker/update_kindlegen

new kindlegen sha256 and directory layout