Drop Go 1.4 support

Due to golang.org/x/crypto/ssh requires a method from STD in Go 1.5.

And Go 1.4 is fairly old version for now.

.travis.yml

@@ -1,7 +1,6 @@
 language: go
 
 go:
-  - 1.4
   - 1.5
   - 1.6
   - 1.7

gogs.go

@@ -1,4 +1,4 @@
-// +build go1.4
+// +build go1.5
 
 // Copyright 2014 The Gogs Authors. All rights reserved.
 // Use of this source code is governed by a MIT-style