remove go 1.2 adn 1.3 from travis build; add tip

Author: jrperritt

.travis.yml

@@ -3,10 +3,9 @@ install:
   - go get golang.org/x/crypto/ssh
   - go get -v -tags 'fixtures acceptance' ./...
 go:
-  - 1.2
-  - 1.3
   - 1.4
   - 1.5
+  - tip
 script: script/cibuild
 after_success:
   - go get golang.org/x/tools/cmd/cover