Add virtual environments folders to gitignore

[alonfnt]

Since it is best practice to create virtual environments for each python project. I've thought that it would be nice to have it on gitignore.

[codecov-commenter]

Codecov Report

Merging #251 into master will increase coverage by 0.00%.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #251   +/-   ##
=======================================
  Coverage   98.44%   98.44%           
=======================================
  Files           7        7           
  Lines         385      387    +2     
  Branches       39       39           
=======================================
+ Hits          379      381    +2     
  Misses          3        3           
  Partials        3        3           

Impacted Files	Coverage Δ
bayes_opt/bayesian_optimization.py	99.03% <0.00%> (+0.01%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update ab8ca64...d54cfaa. Read the comment docs.