Test updates for Travis CI

Travis is running Trusty with GPG 2.0.x, which is
much different from 2.1.x.

Add tests for default key signing.

Add test for gpg1/2 in functional.

pgp/gnupg_test.go

@@ -210,6 +210,8 @@ func (s *Gnupg2SignerSuite) SetUpTest(c *C) {
 
 	c.Assert(s.verifier.InitKeyring(), IsNil)
 
+	s.skipDefaultKey = true
+
 	s.SignerSuite.SetUpTest(c)
 }