add maskVars support from yii 2.0.16

[Alexz29]

No description provided.

[petrabarus]

@Alexz29 you might want to update the composer json to lock yii version 2.0.16.

[petrabarus]

Might want to generate the composer.lock as well.

[petrabarus]

The build still fails, though :(

[codecov]

Codecov Report

Merging #2 into master will increase coverage by 0.75%.
The diff coverage is 83.33%.

@@            Coverage Diff             @@
##           master       #2      +/-   ##
==========================================
+ Coverage      75%   75.75%   +0.75%     
==========================================
  Files           1        1              
  Lines          96       99       +3     
==========================================
+ Hits           72       75       +3     
  Misses         24       24

Impacted Files	Coverage Δ
src/JsonFileTarget.php	75.75% <83.33%> (+0.75%)	⬆️

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[petrabarus]

@Alexz29 couple of reviews from me.

1. I think the yii2 requirement should be >=2.0.16 instead of just 2.0.16
2. Might want to add 7.2 and 7.3 as the travis environment.
3. Let's squash the commits into one so the commit history will look pretty.