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
Merge pull request #163 from jamesarosen/james/no-hdbs-compilation

Drop support for Handlebars template compilation