Tweaked dependency

- Made all dependencies required
    Including `jansson` and `libyaml` should not hurt so much as they
    have no runtime dependency.
    Though `docutils` and `libxml2` are a bit heavy as they require
    Python, they will bring a better user experience.

- Applied `brew audit`

- Adjusted `.travis.yml`

.travis.yml

@@ -32,7 +32,7 @@ before_install:
     fi
 
 install:
-  - brew install --HEAD --debug --verbose --with-docutils --with-jansson --with-libyaml ./universal-ctags.rb
+  - brew install --HEAD --debug --verbose ./universal-ctags.rb
 
 script:
   - ctags --version

universal-ctags.rb

@@ -2,19 +2,18 @@ class UniversalCtags < Formula
   desc "Maintained ctags implementation"
   homepage "https://github.com/universal-ctags/ctags"
   head "https://github.com/universal-ctags/ctags.git"
-  option "without-xml", "Compile without libxml2"
   depends_on "autoconf" => :build
   depends_on "automake" => :build
+  depends_on "docutils" => :build
   depends_on "pkg-config" => :build
-  depends_on "docutils" => :optional
-  depends_on "jansson" => :optional
-  depends_on "libyaml" => :optional
-  conflicts_with "ctags", :because => "this formula installs the same executable as the ctags formula"
+  depends_on "jansson"
+  depends_on "libyaml"
+  uses_from_macos "libxml2"
+  conflicts_with "ctags", because: "this formula installs the same executable as the ctags formula"
 
   def install
     opts = []
-    opts << "--disable-xml" if build.without? "xml"
-    if build.with?("docutils") && OS.linux?
+    if OS.linux?
       py_formula = Formula["docutils"].recursive_dependencies.map(&:name).find { |n| n.include?("python") }
       py_ldflags = `#{Formula[py_formula].opt_bin}/python3-config --ldflags`
       opts << "PYTHON_EXTRA_LDFLAGS=#{py_ldflags.chomp}"
@@ -55,6 +54,6 @@ def caveats
       }
     EOS
     system "#{bin}/ctags", "-R", "."
-    assert_match /func.*test\.c/, File.read("tags")
+    assert_match(/func.*test\.c/, File.read("tags"))
   end
 end