Add method to bindings to get OPENSSL_VERSION_TEXT

* This allows you to check that you're binding against the expected
  version of OpenSSL
* Test is pretty basic (just checks to see that the string starts
  with OpenSSL)
diff --git a/tests/bindings/test_openssl.py b/tests/bindings/test_openssl.py
index 9d63722..8704d93 100644
--- a/tests/bindings/test_openssl.py
+++ b/tests/bindings/test_openssl.py
@@ -17,3 +17,6 @@
 class TestOpenSSL(object):
     def test_api_exists(self):
         assert api
+
+    def test_openssl_version_text(self):
+        assert api.openssl_version_text().find("OpenSSL") == 0