Check that GEM_HOME include '/' and doesn't ends with it to prevent exception in split

Gem `spring` sets GEM_HOME to empty string, which causes exception in line: 
`(ENV['GEM_HOME'] && ENV['GEM_HOME'].split(/\//).last.split(/@/).first) ||`

`split` in case of empty string will return nil which not responds to `last`. Also `GEM_HOME` can be set as `'something/'` which will cause exception too after second split.