Refactoring code to conform to ES6 standards

To update filerjs/filer/tests/spec/fs.chmod.spec.js in keeping with modern ES6 standards, variables that were declared using ‘var’ were refactored to be declared using ‘const’ or ‘let’. ‘const’ and ‘let’ variable declarations are desired over ‘var’ declarations because they are more stringent in terms of scoping. The issue can be found here.

The corresponding pull request can be found here.