Set git branch for DM deps in Gemfile

Gemfile

@@ -8,8 +8,11 @@ DATAMAPPER     = SOURCE == :path ? Pathname(__FILE__).dirname.parent : 'http://g
 DM_VERSION     = '~> 1.2.0.rc1'
 DO_VERSION     = '~> 0.10.6'
 DM_DO_ADAPTERS = %w[ sqlite postgres mysql oracle sqlserver ]
+CURRENT_BRANCH = ENV.fetch('GIT_BRANCH', 'master')
 
-gem 'dm-core', DM_VERSION, SOURCE => "#{DATAMAPPER}/dm-core#{REPO_POSTFIX}"
+gem 'dm-core',     DM_VERSION,
+  SOURCE  => "#{DATAMAPPER}/dm-core#{REPO_POSTFIX}",
+  :branch => CURRENT_BRANCH
 
 group :development do
 
@@ -45,18 +48,24 @@ group :datamapper do
       gem "do_#{adapter}", DO_VERSION, do_options.dup
     end
 
-    gem 'dm-do-adapter', DM_VERSION, SOURCE => "#{DATAMAPPER}/dm-do-adapter#{REPO_POSTFIX}"
+    gem 'dm-do-adapter', DM_VERSION,
+      SOURCE  => "#{DATAMAPPER}/dm-do-adapter#{REPO_POSTFIX}",
+      :branch => CURRENT_BRANCH
   end
 
   adapters.each do |adapter|
-    gem "dm-#{adapter}-adapter", DM_VERSION, SOURCE => "#{DATAMAPPER}/dm-#{adapter}-adapter#{REPO_POSTFIX}"
+    gem "dm-#{adapter}-adapter", DM_VERSION,
+      SOURCE  => "#{DATAMAPPER}/dm-#{adapter}-adapter#{REPO_POSTFIX}",
+      :branch => CURRENT_BRANCH
   end
 
   plugins = ENV['PLUGINS'] || ENV['PLUGIN']
   plugins = plugins.to_s.tr(',', ' ').split.push('dm-migrations').uniq
 
   plugins.each do |plugin|
-    gem plugin, DM_VERSION, SOURCE => "#{DATAMAPPER}/#{plugin}#{REPO_POSTFIX}"
+    gem plugin, DM_VERSION,
+      SOURCE  => "#{DATAMAPPER}/#{plugin}#{REPO_POSTFIX}",
+      :branch => CURRENT_BRANCH
   end
 
 end