removed compiler warning

[WARNING] /Users/luigi/var/src/owner/src/main/java/org/aeonbits/owner/PropertiesManager.java: /Users/luigi/var/src/owner/src/main/java/org/aeonbits/owner/PropertiesManager.java uses unchecked or unsafe operations.
[WARNING] /Users/luigi/var/src/owner/src/main/java/org/aeonbits/owner/PropertiesManager.java: Recompile with -Xlint:unchecked for details.