Fix downloading yarn version from GitHub.

[playpauseandstop]

vagga attempt to download yarn by yarn-version leads to,

ERROR:vagga::capsule::download: Error downloading "https://github.com/yarnpkg/yarn/releases/download/0.27.5/yarn-0.27.5.tar.gz", wget Exited(8)
ERROR:vagga::builder: Error building container "frontend": step YarnDependencies { dir: ".", production: false, optional: true } failed: Error downloading file "/vagga/cache/downloads/c5ee0b78-yarn-0.27.5.tar.gz" from ["https://github.com/yarnpkg/yarn/releases/download/0.27.5/yarn-0.27.5.tar.gz"]
ERROR:vagga::wrapper: Error executing _build: Builder exited with code 1

Cause there is no,

https://github.com/yarnpkg/yarn/releases/download/0.27.5/yarn-0.27.5.tar.gz

as yarn 0.27.5 available at,

https://github.com/yarnpkg/yarn/releases/download/v0.27.5/yarn-v0.27.5.tar.gz

This one-liner fixed it.

---

BTW, this PR not fixed an issue with installing latest Yarn version,

ERROR:vagga::capsule::download: Error downloading "https://github.com/yarnpkg/yarn/releases/download/latest/yarn-latest.tar.gz", wget Exited(8)
ERROR:vagga::builder: Error building container "frontend": step YarnDependencies { dir: ".", production: false, optional: true } failed: Error downloading file "/vagga/cache/downloads/cc383185-yarn-latest.tar.gz" from ["https://github.com/yarnpkg/yarn/releases/download/latest/yarn-latest.tar.gz"]

I'm not sure that logic behind downloading latest yarn version is proper one.

[playpauseandstop]

Closing as invalid, needed to specify yarn version started with v in vagga.yaml. My bad.

[tailhook]

I think we should support both forms, and maybe validate version better.