Add support for installing assets of tinymce-rails-langs

- Remove the ASSETS constant.
- The path to assets should be provided when initialize an new instance of
  AssetInstaller.
Automatically reload TinyMCE configuration file when changed

The config/tinymce.yml will only be reloaded when config.cache_classes
is set to false (so reloading is disabled in production mode by default).
Update license
Update README with TinyMCE 4 examples
Update TinyMCE to 4.0.1
Update to TinyMCE 4.0.2
Include unminified TinyMCE source to prevent precompile from stalling
Update sandbox config
Update to TinyMCE 4.0.6
Update to TinyMCE 4.0.7
Replace deprecated RSpec stub syntax
Update to TinyMCE 4.0.8
Update to TinyMCE 4.0.10
Update to TinyMCE 4.0.11
Update to TinyMCE 4.0.12
Update to TinyMCE 4.0.16
Include form builder example in README
Skip enforcement of available locales
Test against latest Rails releases
Update .travis.yml

add current ruby version 2.1.0 to travis ci