Merge branch 'bump-go-1.8' into 'master'

Bump Go version in RPi image

See merge request !41

docker/Dockerfile_rpi_8

@@ -62,7 +62,7 @@ RUN curl -fsSL "https://www.kernel.org/pub/software/scm/git/git-${GIT_VERSION}.t
   && make all \
   && make install
-ENV GO_VERSION 1.6.2
+ENV GO_VERSION 1.8.1
 RUN curl -fsSL "https://storage.googleapis.com/golang/go${GO_VERSION}.linux-armv6l.tar.gz" \
 	| tar -xzC /usr/local \
   && ln -sf /usr/local/go/bin/go /usr/local/go/bin/gofmt /usr/local/go/bin/godoc /usr/local/bin/