add spring and i18nema for development mode speedup

Just a prototype branch of a possible speedup for development environment. Adjusts development mode to load Spring (default in Rails 4.1), i18nema (fast i18n backend), and English-only locale.
Probably needs more tweaks before it can be merged, but I'm creating this pull request for others to test/comment on the approach.

Compare:
**finished** branch:
```
/dashboard$ time rails runner 'I18n.t("test")'
[deprecated] I18n.enforce_available_locales will default to true in the future. If you really want to skip validation of your locale you can set I18n.enforce_available_locales = false to avoid this message.

real	0m7.018s
user	0m5.892s
sys	0m0.602s
```
**dev_locale_speedup** branch:
```
/dashboard$ time spring rails runner 'I18n.t("test")'

real	0m0.646s
user	0m0.084s
sys	0m0.042s
```