fix(system): improve hasInstance check

[acburdine]

refs #936

• check if name matches global config name as well

The problem (I think) is that if you delete the contents of a ghost install directory without running ghost uninstall, the entry in ~/.ghost/config never gets removed, and thus if you attempt to install Ghost in the same directory again, the hasInstance check would happily report that the instance had already been set up. This expands the check to also check the name field.

Note: upon further thought this may only fix half of the issue - may need a follow-up PR to expand the check in the isSetup getter in the instance class.

[coveralls]

Coverage remained the same at 100.0% when pulling 51828d9 on acburdine:fix/system-hasinstance into 527104b on TryGhost:master.