Improve bootstrap, build and test scripts

Old `script/bootstrap` & `script/build` were unfriendly to Go newbies
because they require the user having GOPATH set in their environment,
and either `godep` installed or `hg` to be able to fetch godep.

However, since dependencies are vendored, we don't have any real build
dependencies except Go itself.

- `script/bootstrap` now checks Go and installs Ruby test bundle
- `script/build` skips compiling if binary is up to date
- `script/test` runs both Go and Cucumber test suites