Remove Trailing Whitespace in User-Agent

After removed, the User-Agent shows in log like this:

[debug] http.go:160 https://index.docker.io/v1/repositories/busybox/images --
HEADERS: map[User-Agent:[docker/0.11.1-dev go/go1.2.2 git-commit/8887e00-dirty kernel/3.14.3-n1 os/linux arch/amd64]]

The code also moved all validation work into validVersion,
to keep the main logic as clean.

Docker-DCO-1.1-Signed-off-by: Derek <crq@kernel.org> (github: crquan)
Merge pull request #5861 from crquan/fix-user-agent-trailing-space

Remove Trailing Whitespace in User-Agent