Avoid bundle clean before caching
Avoid bundle clean before caching

I'm not sure cause, but due to the influence of `gem clean`, the expected
gem seems not to be installed correctly.
In order to avoid a test failure due to this, I fixed that `gem clean` not be executed.
Ref: https://github.com/travis-ci/travis-ci/issues/2518#issuecomment-121168856

This is a workaround. If Travis fixes something, please revert this.

yawboakye/rails,mechanicles/rails,georgeclaghorn/rails,repinel/rails,aditya-kapoor/rails,deraru/rails,arunagw/rails,travisofthenorth/rails,illacceptanything/illacceptanything,yhirano55/rails,Erol/rails,illacceptanything/illacceptanything,deraru/rails,Stellenticket/rails,bogdanvlviv/rails,printercu/rails,travisofthenorth/rails,Erol/rails,kmcphillips/rails,baerjam/rails,Envek/rails,notapatch/rails,jeremy/rails,gauravtiwari/rails,Stellenticket/rails,untidy-hair/rails,schuetzm/rails,palkan/rails,Edouard-chin/rails,aditya-kapoor/rails,vipulnsward/rails,shioyama/rails,joonyou/rails,rails/rails,iainbeeston/rails,schuetzm/rails,tgxworld/rails,lcreid/rails,BlakeWilliams/rails,yalab/rails,tjschuck/rails,vipulnsward/rails,jeremy/rails,illacceptanything/illacceptanything,notapatch/rails,illacceptanything/illacceptanything,mechanicles/rails,kmcphillips/rails,lcreid/rails,palkan/rails,utilum/rails,schuetzm/rails,tgxworld/rails,illacceptanything/illacceptanything,jeremy/rails,baerjam/rails,esparta/rails,assain/rails,illacceptanything/illacceptanything,eileencodes/rails,esparta/rails,pvalena/rails,tjschuck/rails,Erol/rails,kddeisz/rails,aditya-kapoor/rails,vipulnsward/rails,utilum/rails,kddeisz/rails,illacceptanything/illacceptanything,kddeisz/rails,palkan/rails,joonyou/rails,assain/rails,georgeclaghorn/rails,rafaelfranca/omg-rails,assain/rails,printercu/rails,yahonda/rails,Stellenticket/rails,tjschuck/rails,deraru/rails,joonyou/rails,starknx/rails,flanger001/rails,gauravtiwari/rails,yawboakye/rails,tjschuck/rails,flanger001/rails,arunagw/rails,fabianoleittes/rails,shioyama/rails,repinel/rails,palkan/rails,deraru/rails,assain/rails,EmmaB/rails-1,gauravtiwari/rails,pvalena/rails,bogdanvlviv/rails,illacceptanything/illacceptanything,Envek/rails,arunagw/rails,utilum/rails,iainbeeston/rails,rails/rails,bogdanvlviv/rails,fabianoleittes/rails,Vasfed/rails,yhirano55/rails,travisofthenorth/rails,eileencodes/rails,kddeisz/rails,mohitnatoo/rails,EmmaB/rails-1,travisofthenorth/rails,arunagw/rails,Stellenticket/rails,notapatch/rails,yawboakye/rails,tgxworld/rails,fabianoleittes/rails,yalab/rails,flanger001/rails,schuetzm/rails,illacceptanything/illacceptanything,repinel/rails,untidy-hair/rails,Vasfed/rails,illacceptanything/illacceptanything,mechanicles/rails,rails/rails,joonyou/rails,MSP-Greg/rails,BlakeWilliams/rails,gfvcastro/rails,yahonda/rails,rafaelfranca/omg-rails,pvalena/rails,tgxworld/rails,bogdanvlviv/rails,betesh/rails,prathamesh-sonpatki/rails,lcreid/rails,yhirano55/rails,yawboakye/rails,illacceptanything/illacceptanything,pvalena/rails,baerjam/rails,esparta/rails,illacceptanything/illacceptanything,untidy-hair/rails,georgeclaghorn/rails,mechanicles/rails,BlakeWilliams/rails,eileencodes/rails,yahonda/rails,Vasfed/rails,prathamesh-sonpatki/rails,georgeclaghorn/rails,vipulnsward/rails,Vasfed/rails,gfvcastro/rails,iainbeeston/rails,betesh/rails,printercu/rails,mohitnatoo/rails,iainbeeston/rails,aditya-kapoor/rails,utilum/rails,yalab/rails,gfvcastro/rails,yalab/rails,fabianoleittes/rails,betesh/rails,mohitnatoo/rails,kmcphillips/rails,lcreid/rails,starknx/rails,yhirano55/rails,EmmaB/rails-1,gfvcastro/rails,starknx/rails,mohitnatoo/rails,prathamesh-sonpatki/rails,shioyama/rails,illacceptanything/illacceptanything,BlakeWilliams/rails,MSP-Greg/rails,yahonda/rails,Erol/rails,Edouard-chin/rails,flanger001/rails,baerjam/rails,rafaelfranca/omg-rails,MSP-Greg/rails,eileencodes/rails,untidy-hair/rails,printercu/rails,jeremy/rails,Envek/rails,MSP-Greg/rails,betesh/rails,esparta/rails,Envek/rails,illacceptanything/illacceptanything,rails/rails,Edouard-chin/rails,repinel/rails,illacceptanything/illacceptanything,prathamesh-sonpatki/rails,Edouard-chin/rails,notapatch/rails,kmcphillips/rails,shioyama/rails