Install ruby automatically if rbenv is available

People may use other ways to provide the right Ruby version but if they
use rbenv then we can use it automatically.

script/rbenv-install.sh

@@ -2,6 +2,10 @@
 #
 # Install our selected Ruby version defined in the .ruby-version file.
 #
+# Requires:
+# - [rbenv](https://github.com/rbenv/rbenv#readme)
+# - [ruby-build](https://github.com/rbenv/ruby-build#readme)
+#
 # If our ruby-build version is outdated and it can't build the version we want
 # then we try upgrading ruby-build and installing again.
 

script/setup

@@ -17,11 +17,14 @@ YELLOW='\033[0;33m'
 RED='\033[0;31m'
 NO_COLOR='\033[0m'
 
-RUBY_VERSION=$(cat .ruby-version)
-
 # Check ruby version
-if ! ruby --version | grep $RUBY_VERSION > /dev/null; then
-  printf "${RED}Open Food Network requires ruby ${RUBY_VERSION}${NO_COLOR}. Have a look at: https://github.com/rbenv/rbenv\n"
+if command -v rbenv > /dev/null; then
+  ./script/rbenv-install.sh
+elif ! ruby --version | grep $RUBY_VERSION > /dev/null; then
+  RUBY_VERSION=$(cat .ruby-version)
+
+  printf "${RED}Open Food Network requires ruby ${RUBY_VERSION}${NO_COLOR}. "
+  printf "Have a look at: https://github.com/rbenv/rbenv\n"
   exit 1
 fi