Add push trigger and update ci files for github action

[ziyiwu9494]

Update all ci scripts for travis and remove unused variables. Add push trigger for release branch.

[codecov]

Codecov Report

Merging #2237 (56a78d2) into release-2020.06 (c560585) will decrease coverage by 0.00%.
The diff coverage is 88.97%.

@@                 Coverage Diff                 @@
##           release-2020.06    #2237      +/-   ##
===================================================
- Coverage            90.68%   90.67%   -0.01%     
===================================================
  Files                  213      213              
  Lines                11106    11291     +185     
  Branches              1342     1379      +37     
===================================================
+ Hits                 10071    10238     +167     
- Misses                 780      793      +13     
- Partials               255      260       +5     

Impacted Files	Coverage Δ
src/garage/sampler/utils.py	86.79% <ø> (-0.25%)	⬇️
src/garage/tf/algos/npo.py	96.00% <ø> (-0.02%)	⬇️
src/garage/tf/algos/reps.py	96.63% <ø> (-0.02%)	⬇️
src/garage/tf/embeddings/encoder.py	100.00% <ø> (ø)
src/garage/torch/_functions.py	96.36% <ø> (ø)
...c/garage/tf/q_functions/discrete_cnn_q_function.py	96.00% <25.00%> (-4.00%)	⬇️
src/garage/tf/algos/ddpg.py	94.73% <50.00%> (-0.48%)	⬇️
src/garage/tf/algos/dqn.py	94.53% <50.00%> (-0.71%)	⬇️
src/garage/tf/algos/td3.py	97.04% <50.00%> (-0.47%)	⬇️
src/garage/torch/algos/ddpg.py	93.61% <50.00%> (-0.63%)	⬇️
... and 64 more

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