Add handling to modify swagger.json for Travis CI

[kazuhitoyokoi]

• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)

Proposed changes

Currently, the petstore.swagger.io server on the Internet provides the https protocol only. On the other hand, the Docker image, swaggerapi/petstore supports the http protocol only. Because the defaults generated client code uses https, tests using the Docker image on Travis CI failed. To solve the problem, I added handling to change https to http on Travis CI only.

(Previously, the petstore.swagger.io server seemed to provide both https and http. Therefore, there ware no problem in the same change.)

Checklist

• I have read the contribution guidelines
• For non-bugfix PRs, I have discussed this change on the mailing list/slack team.
• I have run grunt to verify the unit tests pass
• I have added suitable unit tests to cover the new/changed functionality

[kazuhitoyokoi]

This fix was moved to #114.