Use standard library's "context" package

[exklamationmark]

While going through the source code, I noticed that it's still using golang.org/x/net/context.

This could be swapped with golang.org/pkg/context without any further code changes. However, this mean the package would need Go 1.7+ to work.

wdyt?

[coveralls]

Coverage remained the same at 82.129% when pulling a5f1028 on exklamationmark:refactor/context into 3e619b6 on graphql-go:master.

[exklamationmark]

This should address #212

[coveralls]

Coverage remained the same at 82.129% when pulling d46ea7e on exklamationmark:refactor/context into 096f5bc on graphql-go:master.

[chris-ramon]

lgtm 👍 — thanks a lot @exklamationmark

• we are running travis tests against go1.7 as lower supported Go version: https://github.com/graphql-go/graphql/blob/master/.travis.yml#L4
• also if just created a release for < go1.7 users: https://github.com/graphql-go/graphql/releases/tag/v0.7.0