Remove `frameworkVersion` as a mandatory field

Move the checks for valid frameworkVersions to only apply when app
`has_location?`
Remove `frameworkVersion` as required from spec
Add missing fields helper, add new validation
Refactor tests, added tests for frameworkVersion
Remove CONST
Define and use file existence helper
Add error for non-existing location
Define and use helper for checking if there is app.js
Fix specs
Reuse existing messaging for missing location
Add test to check for missing location
Remove additional whitespace
Merge pull request #46 from zendesk/trav/make_framework_version_optional

Remove `frameworkVersion` as a mandatory field