Do not delete whole of cache dir.

Previous buildpacks may have files installed in cache dir.
Clearing out the whole cache will mean removing those files too.
We will only clear out cache contents that our buildpack
installed.

bin/compile

@@ -22,9 +22,9 @@ heroku_dir=$build_dir/.heroku
 source ${build_pack_dir}/lib/common.sh
 source ${build_pack_dir}/lib/build.sh
 
-load_previous_npm_node_versions
 
 head "Loading configuration and environment"
+load_previous_npm_node_versions
 load_config
 export_config_vars
 export_mix_env

lib/build.sh

@@ -1,8 +1,10 @@
 cleanup_cache() {
-  if [ $clean_cache ]; then
-    info "`clean_cache` option set to true."
+  if [ $clean_cache = true ]; then
+    info "clean_cache option set to true."
     info "Cleaning out cache contents"
-    rm -rf $cache_dir/*
+    rm -rf $cache_dir/npm-version
+    rm -rf $cache_dir/node-version
+    cleanup_old_node
   fi
 }
 
@@ -21,7 +23,6 @@ download_node() {
   if [ ! -f ${cached_node} ]; then
     info "Downloading node ${node_version}..."
     curl -s ${node_url} -o ${cached_node}
-    cleanup_old_node
   else
     info "Using cached node ${node_version}..."
   fi
@@ -33,7 +34,7 @@ cleanup_old_node() {
   # Note that $old_node will have a format of "v5.5.0" while $node_version
   # has the format "5.6.0"
 
-  if [ $old_node != v$node_version ] && [ -f $old_node_dir ]; then
+  if [ $clean_cache = true ] || [ $old_node != v$node_version ] && [ -f $old_node_dir ]; then
     info "Cleaning up old Node $old_node and old dependencies in cache"
     rm $old_node_dir
     rm -rf $cache_dir/node_modules