Updated url, version, and sha256 references

1. Updated homepage, url, and head to refer to GitHub rather than coin-or.org
2. Updated version reference to most recent (1.17.6)
3. Updated sha256 for appropriate version (pending)

Changes related to the issue discussed at coin-or/CoinUtils#151

clp.rb

@@ -1,9 +1,10 @@
 class Clp < Formula
   desc "Linear programming solver"
-  homepage "https://projects.coin-or.org/Clp"
-  url "https://www.coin-or.org/download/pkgsource/Clp/Clp-1.17.3.tgz"
-  sha256 "c68fd7ed1cba58993d21ba61298366b2d26c6420ba4dbd470d38c3f617316e93"
-  head "https://projects.coin-or.org/svn/Clp/trunk"
+  homepage "https://github.com/coin-or/Clp"
+  url "https://github.com/coin-or/Clp/archive/releases/1.17.6.tar.gz"
+  sha256 ""
+  head "https://github.com/coin-or/Clp/tree/releases/1.17.6/Clp"
+  revision 1
 
   option "with-glpk", "Build with support for reading AMPL/GMPL models"