Bomb out with Bash < 4

Ref #96

abra

@@ -390,6 +390,13 @@ fi
 
 ###### Safety checks
 
+require_bash_4() {
+  # we're using things like `mapfile` which require bash 4+
+  if ! bash -c '[[ $BASH_VERSION > 4.0 ]]'; then
+    error "bash version '$BASH_VERSION' is too old, 4 or newer required"
+  fi
+}
+
 require_pwqgen() {
   if ! type pwqgen > /dev/null 2>&1; then
     error "pwqgen program is not installed"
@@ -406,12 +413,6 @@ require_abra_dir() {
   mkdir -p "$ABRA_DIR"
 }
 
-require_stack() {
-  if [ -z "$STACK_NAME" ]; then
-    error "no stack_name, export \$STACK_NAME=my_cool_app"
-  fi
-}
-
 require_app_latest() {
   APP="$1"
   APP_DIR="$ABRA_DIR/apps/$APP"
@@ -1634,6 +1635,8 @@ sub_network() {
 #######################################
 
 abra() {
+  require_bash_4
+
   # TODO (3wc): we either need to do this, or add 'shellcheck disable' all over
   # the place to handle the dynamically-defined vars
   declare abra___stack abra___env abra__command_ abra__args_ \