Update exercises to use const and let instead of var  #229

We are gradually migrating from var to const. The exercises still contain code that uses var. We should change var usage to const or let as applicable.

PRs welcome :)
Please create one PR per exercise and a change should be applied to the translations as well.

• variables
• strings
• string-length
• revising-strings
• numbers
• rounding-numbers
• number-to-string
• if-statement
• for-loop
• arrays
• array-filtering
• accessing-array-values
• looping-through-arrays
• objects
• object-properties
• functions
• function-arguments
• scope

NOTE:

• Please consider the problem.md and it's translation files as well
• Create PR against es6 branch