Update composer version requirement

If we specify dev-master, just must depend on sgml-stream-interfaces directly or set your minimum-stability.
Let's specify `^0.2`, which allows every release until `v0.3.0`

Author: hershel-theodore-layton

composer.json

@@ -5,7 +5,7 @@
   "require": {
     "hhvm": "^4.73",
     "hhvm/hsl": "^4.41",
-    "hershel-theodore-layton/sgml-stream-interfaces": "dev-master"
+    "hershel-theodore-layton/sgml-stream-interfaces": "^0.2"
   },
   "require-dev": {
     "hhvm/hhvm-autoload": "^3.2",