Remove the peer dependency.

[lijunle]

It is blocking npm shrinkwrap (dylang/npm-check#108) caused by NPM bug (npm/npm#10836).

However, keep #130 open to find a better solution for optional peer dependencies management.

[codecov-io]

Current coverage is 96.31%

Merging #133 into master will not affect coverage as of 044640e

@@            master    #133   diff @@
======================================
  Files           24      24       
  Stmts          515     515       
  Branches       101     101       
  Methods          0       0       
======================================
  Hit            496     496       
  Partial         10      10       
  Missed           9       9       

Review entire Coverage Diff as of 044640e

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