Fixed imports on Examples on readme.

[DarylRodrigo]

When installing the genrl library and tried to run the example programs there were some issues.

• tensorflow-tensorboard==1.5.1 was causing issues with the tensorboard==1.15.0 library. This is most likely due to the fact that tensorflow-tensorboard has been deprecated. This is the reason why I've removed it from the requirements.txt
• Imports for example programs weren't working, so suggested changes (they have been tested).

[codecov[bot]]

Codecov Report

Merging #280 into master will increase coverage by 0.03%. The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #280      +/-   ##
==========================================
+ Coverage   89.62%   89.65%   +0.03%     
==========================================
  Files          87       87              
  Lines        3748     3752       +4     
==========================================
+ Hits         3359     3364       +5     
+ Misses        389      388       -1     

Impacted Files	Coverage Δ
genrl/trainers/base.py	88.23% <ø> (ø)
genrl/trainers/classical.py	86.00% <ø> (ø)
genrl/trainers/onpolicy.py	92.00% <0.00%> (ø)
genrl/environments/vec_env/vector_envs.py	92.46% <0.00%> (+0.21%)	:arrow_up:
genrl/agents/classical/sarsa/sarsa.py	100.00% <0.00%> (+3.84%)	:arrow_up:

[Sharad24]

Thanks for the PR!