fixed install github_flavored_markdown package (avelino#2290)

Author: avelino

Dockerfile

@@ -1,7 +1,7 @@
 FROM golang:alpine
 
 RUN apk add --update -t build-deps curl go git libc-dev gcc libgcc
-RUN go get github.com/russross/blackfriday github.com/gorilla/mux
+RUN go get -u -v github.com/shurcooL/github_flavored_markdown/... github.com/gorilla/mux
 
 WORKDIR /srv