install_spec.rb: Disable test for Linuxbrew
install_spec.rb: Disable test for Linuxbrew

Fixes spurious errors
See #751

Closes Linuxbrew/brew#754.

Signed-off-by: Michka Popoff <7b0496f66f66ee22a38826c310c38b415671b832@gmail.com>

maxim-belkin/brew,Linuxbrew/brew,maxim-belkin/brew,Linuxbrew/brew,Linuxbrew/brew,Linuxbrew/brew,maxim-belkin/brew