feat: Allow triggering a release via the contrib/release script

- Requires docker, git, and package_cloud gems
- Requires the PACKAGECLOUD_API_TOKEN environment variable
- Builds both a deb and rpm, pushing both to packagecloud
- Upgrades the golang build environment to 1.7.5
- Combines both build Dockerfiles into a single, generic env
- Fixes issue where internal-functions file for tar plugin was not executable
- Ensures that built plugins have a callback to clean up their source files before packaging
- Adds support for “betafish” releases, which are untested versions of Dokku that are available via a separate packagecloud repository.

Makefile

@@ -8,6 +8,7 @@ PREBUILT_STACK_URL ?= gliderlabs/herokuish:latest
 DOKKU_LIB_ROOT ?= /var/lib/dokku
 PLUGINS_PATH ?= ${DOKKU_LIB_ROOT}/plugins
 CORE_PLUGINS_PATH ?= ${DOKKU_LIB_ROOT}/core-plugins
+PLUGIN_MAKE_TARGET ?= build-in-docker
 
 # If the first argument is "vagrant-dokku"...
 ifeq (vagrant-dokku,$(firstword $(MAKECMDGOALS)))
@@ -56,7 +57,7 @@ go-build:
 	basedir=$(PWD); \
 	for dir in plugins/*; do \
 		if [ -e $$dir/Makefile ]; then \
-			$(MAKE) -e -C $$dir || exit $$? ;\
+			$(MAKE) -e -C $$dir $(PLUGIN_MAKE_TARGET) || exit $$? ;\
 		fi ;\
 	done