fix for [BUG] #3135 mp4/mov animations can't be opened with QuickTime or Safari

[liasiegelman]

specifies the codec so that the movie can be opened by QuickTime player

Fix #3135

[codecov]

Codecov Report

Merging #3209 (a209a91) into master (93825e2) will not change coverage.
The diff coverage is 0.00%.

@@           Coverage Diff           @@
##           master    #3209   +/-   ##
=======================================
  Coverage   65.50%   65.50%           
=======================================
  Files          25       25           
  Lines        6454     6454           
=======================================
  Hits         4228     4228           
  Misses       2226     2226           

Impacted Files	Coverage Δ
src/animation.jl	38.20% <0.00%> (ø)
src/backends/pgfplotsx.jl	60.89% <0.00%> (ø)

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[BeastyBlacksmith]

Thanks!