[BUGFIX] Restore "required" field attribute functionality

Fixes: #185
Merge pull request #191 from NamelessCoder/requiredfield

[BUGFIX] Restore "required" field attribute functionality
[BUGFIX] Clear cached autoload manifest files on cache clear

Fixes: #189
Merge pull request #190 from NamelessCoder/clearcache

[BUGFIX] Clear cached autoload manifest files on cache clear
[TASK] Use EXT:builder syntax checkers in Travis script