Drop assertthat and dplyr dependencies

[florisvdh]

Fewer dependencies will speed up installation and loading; assertthat and dplyr were almost not used.

The assert_that() precondition in human_filesize() could be dropped IMO because it is an internal function.

[codecov-commenter]

Codecov Report

Merging #40 into master will increase coverage by 0.02%.
The diff coverage is 0.00%.

@@            Coverage Diff            @@
##           master     #40      +/-   ##
=========================================
+ Coverage    5.75%   5.77%   +0.02%     
=========================================
  Files           7       7              
  Lines        1113    1108       -5     
=========================================
  Hits           64      64              
+ Misses       1049    1044       -5     

Impacted Files	Coverage Δ
R/ZenodoRecord.R	0.00% <0.00%> (ø)
R/zen4R_downloader.R	0.00% <0.00%> (ø)

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