Rust version update

[serayuzgur]

It will be better to use rust versions like

rust:
  - stable
  - beta
  - nightly

[codecov]

Codecov Report

Merging #4 into master will increase coverage by 1.72%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master       #4      +/-   ##
==========================================
+ Coverage   82.14%   83.87%   +1.72%     
==========================================
  Files           1        1              
  Lines          28       31       +3     
==========================================
+ Hits           23       26       +3     
  Misses          5        5

Impacted Files	Coverage Δ
src/lib.rs	83.87% <0%> (+1.72%)	⬆️

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[sunjay]

Hi! Thanks for the PR! Could you update the versions to look like the recommended configuration in the documentation? https://docs.travis-ci.com/user/languages/rust/#Choosing-the-Rust-version

It's at the bottom of the section I linked to.

I agree with you that it would be better to list all the versions properly.

[sunjay]

Looks good! Once the build passes this is good to merge.

[sunjay]

Thanks!