Fix syntax error in #4515 on certain shells

Drops the `function` keyword and uses only `func(){}` syntax as per the
POSIX specification:

http://pubs.opengroup.org/onlinepubs/009695399/utilities/xcu_chap02.html#tag_02_09_05

Related: #4531

tools/install.sh

@@ -1,3 +1,4 @@
+main() {
   # Use colors, but only if connected to a terminal, and that terminal
   # supports them.
   if which tput >/dev/null 2>&1; then
@@ -116,3 +117,6 @@ echo 'p.p.s. Get stickers and t-shirts at http://shop.planetargon.com.'
   echo ''
   printf "${NORMAL}"
   env zsh
+}
+
+main