fix backwards compatibility with golang < 1.7

[jeking3]

No description provided.

[googlebot]

Thanks for your pull request. It looks like this may be your first contribution to a Google open source project. Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

📝 Please visit https://cla.developers.google.com/ to sign.

Once you've signed, please reply here (e.g. I signed it!) and we'll verify. Thanks.

---

• If you've already signed a CLA, it's possible we don't have your GitHub username or you're using a different email address. Check your existing CLA data and verify that your email is set on your git commits.
• If your company signed a CLA, they designated a Point of Contact who decides which employees are authorized to participate. You may need to contact the Point of Contact for your company and ask to be added to the group of authorized contributors. If you don't know who your Point of Contact is, direct the project maintainer to go/cla#troubleshoot.
• In order to pass this check, please resolve this problem and have the pull request author add another comment and the bot will run again.

[jeking3]

I signed it!

…

On Thu, Oct 19, 2017 at 8:33 AM, googlebot ***@***.***> wrote: Thanks for your pull request. It looks like this may be your first contribution to a Google open source project. Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA). 📝 *Please visit https://cla.developers.google.com/ <https://cla.developers.google.com/> to sign.* Once you've signed, please reply here (e.g. I signed it!) and we'll verify. Thanks. ------------------------------ - If you've already signed a CLA, it's possible we don't have your GitHub username or you're using a different email address. Check your existing CLA data <https://cla.developers.google.com/clas> and verify that your email is set on your git commits <https://help.github.com/articles/setting-your-email-in-git/>. - If your company signed a CLA, they designated a Point of Contact who decides which employees are authorized to participate. You may need to contact the Point of Contact for your company and ask to be added to the group of authorized contributors. If you don't know who your Point of Contact is, direct the project maintainer to go/cla#troubleshoot. - In order to pass this check, please resolve this problem and have the pull request author add another comment and the bot will run again. — You are receiving this because you authored the thread. Reply to this email directly, view it on GitHub <#118 (comment)>, or mute the thread <https://github.com/notifications/unsubscribe-auth/ALOdbX9r8YwOhmZsUwp3o42rpO3x5m1rks5st0GtgaJpZM4P_J8V> .

[googlebot]

CLAs look good, thanks!

[jeking3]

For reference, this change fixed the Apache Thrift CI builds which run on Go 1.4.2, 1.6.2, and 1.8.3 in containers. Without this change, only the 1.8.3 build worked. With this change, golang.org/x/net/context is must be "go"tten as part of the build process.