Workaround regression in recent rugged versions

Recent versions of rugged are not accepting Rugged::Commit as an
argument to Rugged::Walker#push. We can workaround this by passing
string oid.

Closes mmozuras#15
Refs. libgit2/rugged#860

lib/poper/runner.rb

@@ -32,7 +32,7 @@ def rules
     def commits
       @commits ||= begin
         walker.reset
-        walker.push(@repo.last_commit)
+        walker.push(@repo.last_commit.oid)
         walker.take_while { |c| c.oid != @commit.oid } << @commit
       end
     end