CI: fix 32-bit Linux build failure on Azure CI runs
CI: fix 32-bit Linux build failure on Azure CI runs

Azure uses Ubuntu Bionic (16.04 LTS) including backports;
that now picks up gcc-7 in addition to gcc-5, and by default builds
with gcc-7. Only gfortran-5 is available though, not gfortran-7.

So explicitly use gcc-5 and g++-5

perimosocordiae/scipy,grlee77/scipy,WarrenWeckesser/scipy,Stefan-Endres/scipy,rgommers/scipy,anntzer/scipy,WarrenWeckesser/scipy,andyfaff/scipy,Eric89GXL/scipy,ilayn/scipy,nmayorov/scipy,rgommers/scipy,nmayorov/scipy,anntzer/scipy,endolith/scipy,WarrenWeckesser/scipy,scipy/scipy,zerothi/scipy,andyfaff/scipy,ilayn/scipy,andyfaff/scipy,nmayorov/scipy,scipy/scipy,Stefan-Endres/scipy,mdhaber/scipy,matthew-brett/scipy,perimosocordiae/scipy,Stefan-Endres/scipy,nmayorov/scipy,tylerjereddy/scipy,tylerjereddy/scipy,mdhaber/scipy,perimosocordiae/scipy,e-q/scipy,tylerjereddy/scipy,zerothi/scipy,andyfaff/scipy,Eric89GXL/scipy,mdhaber/scipy,Eric89GXL/scipy,nmayorov/scipy,ilayn/scipy,e-q/scipy,zerothi/scipy,ilayn/scipy,WarrenWeckesser/scipy,Eric89GXL/scipy,WarrenWeckesser/scipy,perimosocordiae/scipy,mdhaber/scipy,scipy/scipy,Eric89GXL/scipy,e-q/scipy,grlee77/scipy,scipy/scipy,perimosocordiae/scipy,tylerjereddy/scipy,vigna/scipy,perimosocordiae/scipy,grlee77/scipy,endolith/scipy,andyfaff/scipy,anntzer/scipy,ilayn/scipy,scipy/scipy,endolith/scipy,vigna/scipy,rgommers/scipy,mdhaber/scipy,matthew-brett/scipy,anntzer/scipy,e-q/scipy,zerothi/scipy,anntzer/scipy,grlee77/scipy,Eric89GXL/scipy,anntzer/scipy,tylerjereddy/scipy,rgommers/scipy,e-q/scipy,WarrenWeckesser/scipy,ilayn/scipy,grlee77/scipy,vigna/scipy,vigna/scipy,matthew-brett/scipy,zerothi/scipy,endolith/scipy,endolith/scipy,zerothi/scipy,Stefan-Endres/scipy,matthew-brett/scipy,Stefan-Endres/scipy,andyfaff/scipy,matthew-brett/scipy,vigna/scipy,rgommers/scipy,scipy/scipy,mdhaber/scipy,endolith/scipy,Stefan-Endres/scipy