Drop support for Handlebars template compilation

 * always use custom template compiler
 * remove `I18N_COMPILE_WITHOUT_HANDLEBARS` feature flag
 * don't run tests with `WITHOUT_HANDLEBARS=true`
 * update dependencies documentation to indicate that only Handlebars-runtime is needed

Resolves #162
Is this true of "the current stable branch"? It's true of master as of this merge, but as of this commit this list will be describing something different from the current state of the repository. I'd suggest the intro line read, "Ember-I18n requires" rather than including the version information.