testing pr: add travis caching directory

[Tlazypanda]

Fixes #8084

Add cache directory which was previously removed to correct the corrupted cache ref #8085

• PR is descriptively titled 📑 and links the original issue above 🔗
• tests pass -- look for a green checkbox ✔️ a few minutes after opening your PR -- or run tests locally with rake test
• code is in uniquely-named feature branch and has no merge conflicts 📁
• screenshots/GIFs are attached 📎 in case of UI updation
• ask @publiclab/reviewers for help, in a comment below

Thanks!

[codecov]

Codecov Report

Merging #8086 into master will decrease coverage by 0.12%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #8086      +/-   ##
==========================================
- Coverage   82.53%   82.41%   -0.13%     
==========================================
  Files          99       99              
  Lines        5737     5737              
==========================================
- Hits         4735     4728       -7     
- Misses       1002     1009       +7     

Impacted Files	Coverage Δ
app/services/execute_search.rb	88.88% <0.00%> (-5.56%)	⬇️
app/api/srch/search.rb	66.24% <0.00%> (-3.83%)	⬇️

[cesswairimu]

🎉