allow setting tilt version via env var

Gemfile

@@ -7,6 +7,7 @@
 # If you have issues with a gem: `bundle install --without-coffee-script`.
 
 RUBY_ENGINE = 'ruby' unless defined? RUBY_ENGINE
+TILT_REPO = "git://github.com/rtomayko/tilt.git"
 
 source :rubygems unless ENV['QUICK']
 gemspec
@@ -14,6 +15,15 @@ gemspec
 gem 'rake'
 gem 'rack-test', '>= 0.5.6'
 
+# Allows stuff like `tilt=1.2.2 bundle install` or `tilt=master ...`.
+# Used by the CI.
+tilt = ENV['tilt'].dup || 'stable'
+tilt.sub! 'tilt-', ''
+if tilt != 'stable'
+  tilt = {:git => TILT_REPO, :branch => tilt} unless tilt =~ /(\d+\.)+\d+/
+  gem 'tilt', tilt
+end
+
 gem 'haml', '>= 3.0', :group => 'haml'
 gem 'builder', :group => 'builder'
 gem 'erubis', :group => 'erubis'