MySQL doesn't work with foreign keys
MySQL doesn't work with foreign keys

This was previously fixed in e84799d but broken in 3f596f8. This commit
reintroduced the conditional that prevents the foreign keys from being
added to MySQL databases.

EmmaB/rails-1,yhirano55/rails,felipecvo/rails,stefanmb/rails,gcourtemanche/rails,MichaelSp/rails,lucasmazza/rails,vipulnsward/rails,joonyou/rails,vipulnsward/rails,flanger001/rails,kmcphillips/rails,flanger001/rails,palkan/rails,utilum/rails,kmayer/rails,lucasmazza/rails,kenta-s/rails,BlakeWilliams/rails,schuetzm/rails,kaspth/rails,illacceptanything/illacceptanything,bogdanvlviv/rails,untidy-hair/rails,gauravtiwari/rails,yahonda/rails,pvalena/rails,sergey-alekseev/rails,gfvcastro/rails,lucasmazza/rails,prathamesh-sonpatki/rails,ttanimichi/rails,yalab/rails,riseshia/railsguides.kr,iainbeeston/rails,mechanicles/rails,arjes/rails,yalab/rails,bogdanvlviv/rails,rafaelfranca/omg-rails,xlymian/rails,pschambacher/rails,esparta/rails,gauravtiwari/rails,aditya-kapoor/rails,riseshia/railsguides.kr,88rabbit/newRails,alecspopa/rails,sealocal/rails,rails/rails,flanger001/rails,rossta/rails,bolek/rails,Erol/rails,odedniv/rails,kenta-s/rails,odedniv/rails,kirs/rails-1,88rabbit/newRails,rossta/rails,eileencodes/rails,deraru/rails,kaspth/rails,mtsmfm/railstest,assain/rails,prathamesh-sonpatki/rails,mechanicles/rails,richseviora/rails,georgeclaghorn/rails,tijwelch/rails,lcreid/rails,untidy-hair/rails,felipecvo/rails,gcourtemanche/rails,rafaelfranca/omg-rails,kirs/rails-1,notapatch/rails,notapatch/rails,eileencodes/rails,voray/rails,mathieujobin/reduced-rails-for-travis,sergey-alekseev/rails,rafaelfranca/omg-rails,rbhitchcock/rails,matrinox/rails,bolek/rails,illacceptanything/illacceptanything,esparta/rails,starknx/rails,hanystudy/rails,bogdanvlviv/rails,mathieujobin/reduced-rails-for-travis,pvalena/rails,samphilipd/rails,travisofthenorth/rails,kmcphillips/rails,illacceptanything/illacceptanything,MichaelSp/rails,MSP-Greg/rails,tjschuck/rails,maicher/rails,utilum/rails,Erol/rails,yasslab/railsguides.jp,esparta/rails,georgeclaghorn/rails,repinel/rails,Envek/rails,georgeclaghorn/rails,printercu/rails,lcreid/rails,yawboakye/rails,MichaelSp/rails,arjes/rails,marklocklear/rails,MSP-Greg/rails,mtsmfm/rails,palkan/rails,betesh/rails,yasslab/railsguides.jp,betesh/rails,gfvcastro/rails,gauravtiwari/rails,riseshia/railsguides.kr,illacceptanything/illacceptanything,Envek/rails,tijwelch/rails,betesh/rails,coreyward/rails,arunagw/rails,illacceptanything/illacceptanything,alecspopa/rails,jeremy/rails,deraru/rails,baerjam/rails,tijwelch/rails,joonyou/rails,mtsmfm/railstest,illacceptanything/illacceptanything,iainbeeston/rails,coreyward/rails,shioyama/rails,yahonda/rails,Sen-Zhang/rails,gfvcastro/rails,Vasfed/rails,deraru/rails,matrinox/rails,pvalena/rails,aditya-kapoor/rails,mijoharas/rails,sergey-alekseev/rails,flanger001/rails,baerjam/rails,sealocal/rails,shioyama/rails,kachick/rails,illacceptanything/illacceptanything,betesh/rails,bradleypriest/rails,Erol/rails,yasslab/railsguides.jp,88rabbit/newRails,repinel/rails,koic/rails,arunagw/rails,yawboakye/rails,yawboakye/rails,eileencodes/rails,rbhitchcock/rails,schuetzm/rails,bogdanvlviv/rails,lcreid/rails,rails/rails,jeremy/rails,utilum/rails,Erol/rails,vipulnsward/rails,baerjam/rails,fabianoleittes/rails,palkan/rails,shioyama/rails,brchristian/rails,baerjam/rails,kmayer/rails,mijoharas/rails,vassilevsky/rails,printercu/rails,esparta/rails,kirs/rails-1,coreyward/rails,prathamesh-sonpatki/rails,ledestin/rails,schuetzm/rails,voray/rails,lcreid/rails,Envek/rails,tgxworld/rails,yalab/rails,kamipo/rails,kamipo/rails,brchristian/rails,amoody2108/TechForJustice,amoody2108/TechForJustice,vipulnsward/rails,tgxworld/rails,joonyou/rails,assain/rails,gavingmiller/rails,xlymian/rails,koic/rails,marklocklear/rails,Sen-Zhang/rails,tjschuck/rails,mohitnatoo/rails,illacceptanything/illacceptanything,ledestin/rails,gavingmiller/rails,illacceptanything/illacceptanything,travisofthenorth/rails,pvalena/rails,tjschuck/rails,pschambacher/rails,Stellenticket/rails,shioyama/rails,vassilevsky/rails,yahonda/rails,Sen-Zhang/rails,kddeisz/rails,samphilipd/rails,matrinox/rails,arunagw/rails,maicher/rails,yawboakye/rails,repinel/rails,sealocal/rails,travisofthenorth/rails,rbhitchcock/rails,Edouard-chin/rails,travisofthenorth/rails,jeremy/rails,vassilevsky/rails,illacceptanything/illacceptanything,elfassy/rails,deraru/rails,mohitnatoo/rails,mtsmfm/rails,joonyou/rails,kaspth/rails,bradleypriest/rails,mechanicles/rails,tgxworld/rails,pschambacher/rails,ngpestelos/rails,eileencodes/rails,aditya-kapoor/rails,koic/rails,EmmaB/rails-1,Vasfed/rails,mtsmfm/railstest,mohitnatoo/rails,brchristian/rails,kddeisz/rails,amoody2108/TechForJustice,arjes/rails,gavingmiller/rails,rails/rails,yalab/rails,BlakeWilliams/rails,illacceptanything/illacceptanything,schuetzm/rails,yhirano55/rails,printercu/rails,notapatch/rails,aditya-kapoor/rails,palkan/rails,kddeisz/rails,kenta-s/rails,Envek/rails,rails/rails,kamipo/rails,Edouard-chin/rails,Spin42/rails,richseviora/rails,ledestin/rails,voray/rails,alecspopa/rails,notapatch/rails,Spin42/rails,iainbeeston/rails,fabianoleittes/rails,yahonda/rails,stefanmb/rails,fabianoleittes/rails,kachick/rails,ngpe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