Fix POSIX compliance on bootstrap
Fix POSIX compliance on bootstrap

`./script/bootstrap`  was failing on linux

```
./script/bootstrap: 8: ./script/bootstrap: [[: not found
```
Fixed by using POSIX compliant `[` instead of `[[`

bkeepers/PRobot,bkeepers/PRobot,pholleran-org/probot,pholleran-org/probot,probot/probot,probot/probot,pholleran-org/probot,probot/probot