New Cask: gfortran (#40065)

* New Cask: gfortran 6.1

* Update gfortran.rb

* add conflicts_with and pkgutil uninstaller

* install Sierra build on Sierra and up

* repeat the url comment

Casks/gfortran.rb

@@ -0,0 +1,33 @@
+cask 'gfortran' do
+  if MacOS.version == :el_capitan
+    version '6.1'
+    sha256 'eb817bce64bf9032595e09166bdaaf740c83bf7258f900b79cd6786437bacbf4'
+
+    # coudert.name/software was verified as official when first introduced to the cask
+    url "http://coudert.name/software/gfortran-#{version}-ElCapitan.dmg"
+    pkg "gfortran-#{version}-ElCapitan/gfortran.pkg"
+  else
+    version '6.3'
+    sha256 '38b81bc878dba41cfdbb0c335aec5a97554a5d1766fb3e3ca6be7da0df9e8e09'
+
+    # coudert.name/software was verified as official when first introduced to the cask
+    url "http://coudert.name/software/gfortran-#{version}-Sierra.dmg"
+    pkg 'gfortran.pkg'
+  end
+
+  name 'gfortran'
+  homepage 'https://gcc.gnu.org/wiki/GFortranBinaries'
+
+  conflicts_with formula: 'gcc'
+  depends_on macos: '>= :el_capitan'
+
+  uninstall delete:  [
+                       '/usr/local/gfortran',
+                       '/usr/local/bin/gfortran',
+                     ],
+            pkgutil: 'com.gnu.gfortran'
+
+  caveats do
+    files_in_usr_local
+  end
+end