twitter-server: Add imartin to OWNERS

Summary: Problem/Solution:

I feel pretty comfortable with TwitterServer code at this point so why not
become an OWNER!

Differential Revision: https://phabricator.twitter.biz/D143927

OWNERS

@@ -1,5 +1,6 @@
 ccoco
 dschobel
+imartin
 jparker
 koliver
 mnakamura