bump minimum tested version to go 1.4

Adding the new BasicAuthTransport raises the minimum required version of
go to 1.4.

.travis.yml

@@ -1,7 +1,7 @@
 language: go
 go:
- - 1.1
- - 1.2
+ - 1.4
+ - 1.5
  - tip
 install: go get -v ./github
 script: go test -v ./github