Improve Travis build

[yanokwa]

Continue with #35

• Make sure links are public
• Fix Windows paths
• Figure out how to zip binaries
• Use git hash to version the binary
• Confirm build status icon works
• Update readme with location of build
• Update to Python 3

[yanokwa]

@shouryaj I think the Windows paths, zips, and git hash could be solved with workspaces.

I'm thinking you build the Windows binary, stick it in a workspace, then build the Mac binary, put that in the same workspace, then use the Mac environment to generate the zip. e.g., ODK-XLSForm-Offline-macOS-$hash.zip. And again, you can hash using git describe --tags --dirty --always.

Does that sound feasible?

[shouryaj]

@yanokwa sounds good, I can work on adding that this week.

[PieterBenjamin]

Hey @yanokwa , I've been working on this with estberg. Can you expand on "Fix windows paths"?