gcc: Revert gcc to revision 1 for Linuxbrew
gcc: Revert gcc to revision 1 for Linuxbrew

isl 0.19 is missing the file include/isl/band.h,
which is required for building GCC 5.5.0.
Use the previous gcc bottle, which was built using isl 0.18.

rwhogg/homebrew-core,Linuxbrew/homebrew-core,LinuxbrewTestBot/homebrew-core,rwhogg/homebrew-core,Linuxbrew/homebrew-core,LinuxbrewTestBot/homebrew-core,maxim-belkin/homebrew-core,maxim-belkin/homebrew-core