bash 5.x support

[PartyImp]

the compatibility check fails for bash 5.x due to minor version check

bash-5.0# bash -x foo.sh 
+ source concurrent.lib.sh
+ concurrent - 'sleep test' sleep 1
+ [[ -z 5 0 0 1 release x86_64-pc-linux-musl ]]
+ [[ 5 -lt 4 ]]
+ [[ 0 -lt 2 ]]
+ __crt__error 'Requires Bash version 4.2 for '\''declare -g'\'' (you have 5.0.0(1)-release)'
+ echo 'ERROR (concurrent): Requires Bash version 4.2 for '\''declare -g'\'' (you have 5.0.0(1)-release)'
ERROR (concurrent): Requires Bash version 4.2 for 'declare -g' (you have 5.0.0(1)-release)
+ exit 1