Adds write permission test on ~/.bash_profile prior to backup attempt

The install script attempts to backup ~/.bash_profile when it does not exist.
```
$ ~/.bash_it/install.sh 
cp: /Users/sgerrand/.bash_profile: No such file or directory
Your original .bash_profile has been backed up to .bash_profile.bak
```
This adds a test for write permission (and implicitly the existence of) on `~/.bash_profile` prior to attempting a backup.