yacas: fix 404.

Closes Homebrew/homebrew#37072.

Signed-off-by: Mike McQuaid <mike@mikemcquaid.com>

Formula/yacas.rb

@@ -1,17 +1,15 @@
-require 'formula'
-
 class Yacas < Formula
-  homepage 'http://yacas.sourceforge.net'
-  url 'http://yacas.sourceforge.net/backups/yacas-1.3.4.tar.gz'
-  sha1 'b0918e7f1e697fde48f09055528dacbf7513b931'
+  homepage "http://yacas.sourceforge.net"
+  url "https://downloads.sourceforge.net/project/yacas/yacas-source/1.3/yacas-1.3.4.tar.gz"
+  sha1 "c166fe92eaf3dd4218e4b10ac752457b980ae6e2"
 
   option "with-server", "Build the network server version"
 
   def install
-    args = [ "--disable-silent-rules",
-             "--disable-dependency-tracking",
-             "--prefix=#{prefix}"
-    ]
+    args = ["--disable-silent-rules",
+            "--disable-dependency-tracking",
+            "--prefix=#{prefix}",
+           ]
 
     args << "--enable-server" if build.with? "server"
 
@@ -19,4 +17,8 @@ class Yacas < Formula
     system "make", "install"
     system "make", "test"
   end
+
+  test do
+    system "#{bin}/yacas", "--version"
+  end
 end