Update get.sh to determine latest full release from Github

This commit updates the one-line installer `get.sh` to use the latest
full release from Github rather than use a hard-coded version string.

Draft releases and prereleases are not returned by the endpoint used to
determine the version.

Signed-off-by: John McCabe <john@johnmccabe.net>

get.sh

@@ -1,4 +1,4 @@
-version=0.3
+version=$(curl -s https://api.github.com/repos/alexellis/faas-cli/releases/latest | grep 'tag_name' | cut -d\" -f4)
 
 hasCli() {
     has=$(which faas-cli)