Upgrade to Go 1.7

[endophage]

Upgrade circle CI to use Go 1.7, fix any breakage that causes (none expected based on release notes).

This should give us faster, smaller binaries. Among other specifically interesting improvements, sha256 has been sped up by 10% or more, and we do a lot of sha256 hashing.

[cyli]

We may have some lint fixes I think for go 1.7. On RC5 I see:

server/handlers/default_test.go:419: literal copies lock value from *metaStore: github.com/docker/notary/server/storage.MemStorage contains sync.Mutex
server/handlers/default_test.go:461: literal copies lock value from *metaStore: github.com/docker/notary/server/storage.MemStorage contains sync.Mutex

[riyazdf]

@cyli: If I remember correctly, I think that vet issue was handled by #793?

[cyli]

I still seem to be seeing it :| It's a mutex in a different file I think

[riyazdf]

ah, yes, you're right. We can make a similar change 👍

[HuKeping]

Hola Go 1.7!