Only install shiny and rmarkdown if needed

Just a tiny build efficiency tweak

rstudio/add_shiny.sh

@@ -14,7 +14,7 @@ if [ "$ADD" == "shiny" ]; then
     wget --no-verbose "https://s3.amazonaws.com/rstudio-shiny-server-os-build/ubuntu-12.04/x86_64/shiny-server-$VERSION-amd64.deb" -O ss-latest.deb && \
     gdebi -n ss-latest.deb && \
     rm -f version.txt ss-latest.deb && \
-    install2.r -e shiny rmarkdown && \
+    install2.r -e --skipinstalled shiny rmarkdown && \
     cp -R /usr/local/lib/R/site-library/shiny/examples/* /srv/shiny-server/ && \
     rm -rf /var/lib/apt/lists/* && \
     mkdir -p /var/log/shiny-server && \