Use Psr\Http\Client\ClientInterface as type

[holtkamp]

As suggested in #32

[holtkamp]

@kocsismate I encountered this PR, could you consider merging it?

[codecov]

Codecov Report

Merging #33 (58ac40e) into master (8d48ee7) will not change coverage.
The diff coverage is 0.00%.

❗ Current head 58ac40e differs from pull request most recent head 7c78944. Consider uploading reports for the commit 7c78944 to get more accurate results

@@            Coverage Diff            @@
##             master      #33   +/-   ##
=========================================
  Coverage     97.92%   97.92%           
  Complexity      514      514           
=========================================
  Files            29       29           
  Lines          1060     1060           
=========================================
  Hits           1038     1038           
  Misses           22       22           

Files Changed	Coverage Δ
src/JsonApi/Client/JsonApiClient.php	0.00% <0.00%> (ø)

[holtkamp]

@kocsismate subtle but shameless bump?

[kocsismate]

@holtkamp OK, I'll try to find some time in the following days to merge your PR and release a new major version!