Fix unbound variable error

```
remote: -----> App not compatible with buildpack: https://buildpack-registry.s3.amazonaws.com/buildpacks/heroku/ruby.tgz        
remote:        /tmp/codon/tmp/buildpacks/50d5eddf222a9b7326028041d4e6509f915ccf2c/bin/detect: line 12: BUILD_DIR: unbound variable        
remote: /tmp/codon/tmp/buildpacks/50d5eddf222a9b7326028041d4e6509f915ccf2c/bin/detect: line 12: output::error: command not found        
```

Author: schneems

bin/detect

@@ -3,21 +3,26 @@
 set -euo pipefail
 
 APP_DIR=$1
+BIN_DIR=$(cd "$(dirname "$0")" || exit; pwd) # absolute path
 
 if [ -f "$APP_DIR/Gemfile" ]; then
   echo "Ruby"
   exit 0
 fi
 
+
+# shellcheck source=bin/support/bash_functions.sh
+source "$BIN_DIR/support/bash_functions.sh"
+
 output::error <<EOF
 Error: Your app is configured to use the Ruby buildpack,
-but we couldn't find any supported Ruby project files ('Gemfile.lock').
+but we couldn't find any supported Ruby project files ('Gemfile' and 'Gemfile.lock').
 
-A Ruby app on Heroku must have a 'Gemfile.lock' in the root directory of its source code.
+A Ruby app on Heroku must have a 'Gemfile' and 'Gemfile.lock' in the root directory of its source code.
 
 Currently the root directory of your app contains:
 
-$(ls -1A --indicator-style=slash "${BUILD_DIR}" || true)
+$(ls -1A --indicator-style=slash "${APP_DIR}" || true)
 
 If your app already has a package manager file, check that it:
 

bin/support/bash_functions.sh

@@ -2,6 +2,30 @@
 
 set -euo pipefail
 
+ANSI_BLUE='\033[1;34m'
+ANSI_RED='\033[1;31m'
+ANSI_YELLOW='\033[1;33m'
+ANSI_RESET='\033[0m'
+
+# Output a styled multi-line error message to stderr.
+#
+# Usage:
+# ```
+# output::error <<-EOF
+# 	Error: The error summary.
+#
+# 	Detailed description.
+# EOF
+# ```
+function output::error() {
+	local line
+	echo >&2
+	while IFS= read -r line; do
+		echo -e "${ANSI_RED} !     ${line}${ANSI_RESET}" >&2
+	done
+	echo >&2
+}
+
 curl_retry_on_18() {
   local ec=18;
   local attempts=0;

spec/hatchet/bugs_spec.rb

@@ -25,4 +25,16 @@
       end
     end
   end
+
+  it "detect fails when no Gemfile is present" do
+    Hatchet::Runner.new("default_ruby", allow_failure: true).tap do |app|
+      app.before_deploy do
+        FileUtils.rm("Gemfile")
+      end
+      app.deploy do |app|
+        expect(app.output).to include("A Ruby app on Heroku must have a 'Gemfile' and 'Gemfile.lock' in the root directory of its source code.")
+        expect(app).not_to be_deployed
+      end
+    end
+  end
 end