Use git clone to fetch shfmt intead of go get
There is an import issue on latest shfmt master branch which prevents
downloading shfmt  via `go get`.
@@ -19,9 +19,10 @@ RUN \
go get github.com/mattn/goveralls && \
go get -u github.com/Masterminds/glide && \
go get golang.org/x/tools/cmd/goimports && \
- go get -u mvdan.cc/sh/cmd/shfmt && \
+ git clone https://github.com/mvdan/sh.git $GOPATH/src/mvdan.cc/sh && \
cd /go/src/mvdan.cc/sh/cmd/shfmt && \
- git checkout v2.3.0 && \
+ git checkout v2.5.0 && \
+ go get mvdan.cc/sh/cmd/shfmt && \
go install && \
go get -u github.com/golang/mock/gomock && \
go get -u github.com/rmohr/mock/mockgen && \
