Use `python2.7` instead of `python2`

Ubuntu has both aliases, but Mac only declares python2.7, not python2.

Test: `m sepolicy_tests` on internal master
Change-Id: Ia2b5d2d95195f307befbc1893a465752f14b8a7b
diff --git a/python/binary.go b/python/binary.go
index 91b7a54..b7b5056 100644
--- a/python/binary.go
+++ b/python/binary.go
@@ -162,7 +162,7 @@
 	var interp string
 	switch actual_version {
 	case pyVersion2:
-		interp = "python2"
+		interp = "python2.7"
 	case pyVersion3:
 		interp = "python3"
 	default: