THRIFT-3177 Fails to run rake test

Patch: István Karaszi
diff --git a/lib/rb/Rakefile b/lib/rb/Rakefile
index 9dc8324..0831e68 100644
--- a/lib/rb/Rakefile
+++ b/lib/rb/Rakefile
@@ -86,7 +86,7 @@
 desc 'Run the compiler tests (requires full thrift checkout)'
 task :test do
   # ensure this is a full thrift checkout and not a tarball of the ruby libs
-  cmd = 'head -1 ../../README 2>/dev/null | grep Thrift >/dev/null 2>/dev/null'
+  cmd = 'head -1 ../../README.md 2>/dev/null | grep Thrift >/dev/null 2>/dev/null'
   system(cmd) or fail "rake test requires a full thrift checkout"
   sh 'make', '-C', File.dirname(__FILE__) + "/../../test/rb", "check"
 end