travis-ci: Add Go 1.2

This PR adds Go 1.2 to travis-ci and fixes test failure with Go 1.2. I'm not sure we can remove Go 1.1 support.

An assertion in s3_test.go raises a failure with Go 1.2 like below:

```
    c.Assert(resp.ContentLength, gocheck.Equals, int64(len("content")))
... obtained int64 = -1
... expected int64 = 7
```

This failure is caused by [changes in net/http in Go 1.2](https://code.google.com/p/go/source/detail?r=3a388be1862ee310ec0036e3bc5bceeaf30ce315&path=/src/pkg/net/http/server.go).