fix: use gnu readlink to make tests pass on os x

.gitignore

@@ -1,4 +1,5 @@
 tests/dokku
 tests/fixtures
 tests/bin/plugn
+tests/bin/readlink
 .vagrant

Makefile

@@ -20,7 +20,15 @@ else
 endif
 endif
 
-ci-dependencies: shellcheck bats
+readlink:
+ifeq ($(shell uname),Darwin)
+ifeq ($(shell greadlink > /dev/null 2>&1 ; echo $$?),127)
+	brew install coreutils
+endif
+	ln -nfs `which greadlink` tests/bin/readlink
+endif
+
+ci-dependencies: shellcheck bats readlink
 
 lint:
 	# these are disabled due to their expansive existence in the codebase. we should clean it up though