Increases strictness of explicit i18n dependency

i18n v0.6.7 introduces I18n.enforce_available_locales=, which Padrino
depends on. Earlier versions of the i18n gem that are on the 0.6 line
will satisfy the gemspec dependency but will not allow the tests to
pass since the gem lacks that method.
rebase string urls to uri_root, closes #1535
Merge pull request #1533 from fj/topic/specify-i18n-dependency

Increase strictness of explicit i18n dependency
Allow mri-2.1 to fail
Use ENV['RACK_BASE_URI'] in padrino-admin store_location!, fixes #1534
Merge branch 'fix-1534'
Enable :except option to :protect_from_csrf.
Introduce #generate_with_parts for padrino-gen test.
The generator should abort if constant name already exists.
make absolute_url for relative path more intuitive
Merge pull request #1537 from padrino/fix-1535

rebase string urls to uri_root, closes #1535
Merge pull request #1539 from padrino/enable-except-option

Enable :except option to :protect_from_csrf.
Merge pull request #1540 from padrino/abort-gen

The generator should abort if constant name already exists.
Use `unless except.empty?` instead of `if !except.empty?`.
Re-implement `AuthenticityToken`.
Change test cases for #1539.
Skip loading models for sequel migration tasks, closes #1520
Allow to use parameter of string, regex, and array.
resolve templates relative to controller name, closes #922
quick fix for requestless things