Upgrade to simulacrum 0.6.1

[mpilquist]

Simulacrum 0.6.1 was just released. Type classes automatically extend Any and Serializable now, so explicit declarations of these super types are not necessary any longer.

[codecov-io]

Current coverage is 89.12%

Merging #820 into master will not affect coverage as of 057d4b8

@@            master    #820   diff @@
======================================
  Files          168     168       
  Stmts         2309    2309       
  Branches        75      75       
  Methods          0       0       
======================================
  Hit           2058    2058       
  Partial          0       0       
  Missed         251     251       

Review entire Coverage Diff as of 057d4b8

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[fthomas]

👍

[milessabin]

👍