Auto protobuf schema generation with Protobuf 2.0.2

[dingxiangfei2009]

Referring to #38, #39 and #40.

This is a merge between #38 and #40. A new change in this PR is bumping protoc-rust version to match the version of protobuf being used.

[psivesely]

When this is ready if you could clean up the git history (mostly the most recent commits) that would be 💯 and I'll close #40.

[dingxiangfei2009]

I just remove the wip commits from this PR.

[psivesely]

You have 3.5.1 in 6 places in 2 files. It would be nice if we could set the protobuf version in a single place. E.g., create a top-level file called protobuf-version.txt that just reads 3.5.1 and then have build.rs and install-protobuf.sh both read that file. Or if you have another idea for how to simply maintain a single point of version declaration.

[dingxiangfei2009]

I have an idea to implement this. I will put libprotoc 3.5.1 string into PROTOC_VERSION file at the project root. build.rs will confirm the version of protoc from the (compile time) PROTOC_VERSION environment variable. I will also set up the Travis CI to load the version file into the environment variable.

[psivesely]

LGTM!

[psivesely]

Why did you get rid of cargo cache? Could you have avoided whatever problem by adding a call to cargo update before building?

[dingxiangfei2009]

Oh yes, my bad. Making a new change now.