Don't test on R 3.3 anymore

[lorenzwalthert]

Closes #801.

[codecov-commenter]

Codecov Report

Merging #804 (16c9339) into master (2985c84) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #804   +/-   ##
=======================================
  Coverage   90.46%   90.46%           
=======================================
  Files          47       47           
  Lines        2433     2433           
=======================================
  Hits         2201     2201           
  Misses        232      232           

Impacted Files	Coverage Δ
R/environments.R	100.00% <ø> (ø)

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[github-actions]

This is how benchmark results would change (along with a 95% confience interval in relative change) if b9b732c is merged into master:

• cache_applying: 0.1s -> 0.1s [-1.69%, +8.05%]
• cache_recording: 1.35s -> 1.35s [-1.38%, +0.93%]
• without_cache: 3.56s -> 3.56s [-0.79%, +0.63%]

Further explanation regarding interpretation and methodology can be found in the documentation.