fix(build): version of Slickgrid-Universal are out of sync, fixes #956

[ghiscoding]

• it doesn't seem to be a good idea to use fixed versions of Slickgrid-Universal within Angular-Slickgrid because users can become out of sync easily, let's go back to using minor version ^

[codecov]

Codecov Report

Merging #957 (b4e0a76) into master (009ecab) will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##            master      #957   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           12        12           
  Lines          979       979           
  Branches       328       328           
=========================================
  Hits           979       979           

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