Update README.md
fallbackLanguage addition

by adding "$translateProvider.fallbackLanguage("en_GB");", the module will now try to get the english translation  if the translation is missing in the used language
Update translateServiceSpec.js

add test for fallback new functionality
merge commit
fix(): fix bower.json
feat(translateProvider): add fallbackLanguage() method

- adds translateProvider.fallbackLanguage(key) to configure fallback language,
if translation id in current language doesn't exist.

Closes #67