deps: update type-is to v1.6.18

[3imed-jaberi]

• https://github.com/jshttp/type-is/blob/master/HISTORY.md#1618--2019-04-26
• https://github.com/jshttp/type-is/blob/master/HISTORY.md#1617--2019-04-25

[codecov]

Codecov Report

Merging #1481 into master will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##            master     #1481   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            4         4           
  Lines          487       487           
  Branches       136       136           
=========================================
  Hits           487       487           

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[3imed-jaberi]

Ref #comment