Fix Gemfile syntax.

Using Ruby 2.1.2 and Passenger 4.0.49, when Redmine starts the following
error appears in the Console:

    stderr: /Users/me/.gem/ruby/2.1.2/gems/bundler-1.7.1/lib/bundler/dsl.rb:35:in `eval_gemfile': Gemfile syntax error compile error (Bundler::GemfileError)
    stderr: /Users/me/Web/ruby/redmine/plugins/redmine_dashboard/Gemfile:8: syntax error, unexpected ':', expecting kEND
    stderr: ... gem 'transifex-ruby', github: 'jgraichen/transifex-ruby', r...
    stderr:                               ^
    stderr: /Users/me/Web/ruby/redmine/plugins/redmine_dashboard/Gemfile:8: syntax error, unexpected ',', expecting kEND
    stderr: ...b: 'jgraichen/transifex-ruby', require: false
    stderr:                               ^
    stderr: /Users/me/Web/ruby/redmine/plugins/redmine_dashboard/Gemfile:9: syntax error, unexpected ':', expecting kEND
    stderr:   gem 'inifile', require: false
    stderr:                          ^
    stderr: 	from /Users/me/.gem/ruby/2.1.2/gems/bundler-1.7.1/lib/bundler/dsl.rb:10:in `evaluate'
    stderr: 	from /Users/me/.gem/ruby/2.1.2/gems/bundler-1.7.1/lib/bundler/definition.rb:25:in `build'
    stderr: 	from /Users/me/.gem/ruby/2.1.2/gems/bundler-1.7.1/lib/bundler.rb:154:in `definition'
    stderr: 	from /Users/me/.gem/ruby/2.1.2/gems/bundler-1.7.1/lib/bundler.rb:117:in `setup'
    stderr: 	from /Users/me/.gem/ruby/2.1.2/gems/bundler-1.7.1/lib/bundler/setup.rb:17

Fix the error by updating the syntax for specifying gem options.