build: target for go library dependencies

Didn't notice it because vendor/ was already populated on my dev
machine, but did notice it in Docker build.

Docker build now consumes devsetup like regular user, so this should
catch future problems.

Remove remaining curl|shit functionality from lazy.sh (no checkout logic
needed anymore).

refs #35

docs/installation.rst

@@ -57,7 +57,7 @@ and serves as a reference for build dependencies and procedure:
     sudo docker run -it --rm \
         -v "${PWD}:/zrepl" \
         --user "$(id -u):$(id -g)" \
-        zrepl_build make release
+        zrepl_build make vendordeps release
 
 Alternatively, you can install build dependencies on your local system and then build in your ``$GOPATH``:
 
@@ -67,7 +67,7 @@ Alternatively, you can install build dependencies on your local system and then
     git clone https://github.com/zrepl/zrepl.git "${GOPATH}/src/github.com/zrepl/zrepl"
     cd "${GOPATH}/src/github.com/zrepl/zrepl"
     ./lazy.sh devsetup
-    make release
+    make vendordeps release
 
 Build results are located in the ``artifacts/`` directory.