Do not ignore validation of referenced schemas

[vearutop]

This PR enables strict schema dereferencing when target is clearly an instance of JSON schema.

Resolves #116.

[codecov]

Codecov Report

Merging #117 (c348881) into master (fa87be2) will increase coverage by 0.07%.
The diff coverage is 100.00%.

@@             Coverage Diff              @@
##             master     #117      +/-   ##
============================================
+ Coverage     92.19%   92.27%   +0.07%     
- Complexity      753      754       +1     
============================================
  Files            27       27              
  Lines          1641     1643       +2     
============================================
+ Hits           1513     1516       +3     
+ Misses          128      127       -1     

Impacted Files	Coverage Δ	Complexity Δ
src/Schema.php	95.92% <100.00%> (+0.16%)	342.00 <0.00> (+1.00)

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