Install curl on image

[bboughton]

Install curl on image to allow oauth2l to use the curl subcommand to make requests authenticated with Google Auth.

Fixes #115

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[googlebot]

Thanks for your pull request. It looks like this may be your first contribution to a Google open source project (if not, look below for help). Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

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

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Corporate signers

• Your company has a Point of Contact who decides which employees are authorized to participate. Ask your POC to be added to the group of authorized contributors. If you don't know who your Point of Contact is, direct the Google project maintainer to go/cla#troubleshoot (Public version).
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[bboughton]

@googlebot I signed it!

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[andyrzhao]

Thanks for the patch Brian!