Fix deployment to use Cloud SDK

[aktech]

Fixes #136

• Fixes broken deployment process.
• Uses gcloud CLI.
• Uses service account for authentication, added the key for the same in travis environment variables and adds the encrypted key in the repo (Using a better way to store encrypted key is the one for later).
• Removed dependency from deploy.py and got rid of it.
• Removed app.yaml.template. Now one app.yaml works for any deployment.
• Updated Readme instructions.
• Tidying up .travis.yml.

Deployment via Travis behaviour:

• Tagged release (promoted to main site)
• Repo branch Pull Requests deployed to branch-name-dot-sympy-gamma-tests.appspot.com

(This uses the Travis's inbuilt deploy support for GAE to simplify deployments: The PR builds skips the deployment altogether, we cannot have pull request number as the version number because the deployment can only happen in the branch push triggered builds.)

Deployed via branch push build: https://fix-deploy-dot-sympy-gamma-hrd.appspot.com

[aktech]

Will merge within 24 hr, if no objections. @asmeurer Let me know if you have any questions and if anything is not clear in the instructions.

[asmeurer]

Is there any way to make it so GitHub shows the deployment on the pull request (GitHub has a feature where it can show deployments above the merge button, for example, like it does for the bot here)

[aktech]

I think you mean this one? asmeurer/GitHub-Issues-Test#43
Surely, we can use the @sympy-bot to implement something like that, the way you have in the PR above.