Add eXist-db.app 3.0 (nightly build) (#3067)

* Add eXist-db.app 3.0 (nightly build)

* Remove nokogiri dependency as instructed

- Replaced nokogiri with io.read.scan as proposed by @claui.
- Addressed all brew cask style concerns
- Tested the resulting script and confirmed that it successfully grabbed the latest nightly

Casks/exist-db-nightly.rb

@@ -0,0 +1,19 @@
+cask 'exist-db-nightly' do
+  version :latest
+  sha256 :no_check
+
+  # static.adamretter.org.uk/exist-nightly was verified as official when first introduced to the cask
+  url do
+    require 'open-uri'
+    base_url = 'http://static.adamretter.org.uk/exist-nightly'
+    builds_url = "#{base_url}/table.html"
+    latest_build_filename = open(builds_url) do |io|
+      io.read.scan(%r{<tr>.*?<td>(.*?)</td>.*?<a href="([^\"]+)">dmg}m).max[1]
+    end
+    "#{base_url}/#{latest_build_filename}"
+  end
+  name 'eXist-db'
+  homepage 'http://exist-db.org/exist/apps/homepage/index.html'
+
+  app 'eXist-db.app'
+end