Adding double quotations for _JAVACMD and CMD values.

gosu script cannot display "gs>" prompt when JAVA_HOME path contains space character.

Steps to reproduce with gosu-0.10.2
1. Set JAVA_HOME to the path with space character from Terminal application on Mac OS X.
$ export JAVA_HOME="/Library/Internet Plug-Ins/JavaAppletPlugin.plugin/Contents/Home"
2. Launch gosu script

// actual result: Error is displayed on the shell script.
$ ./gosu-0.10.2/bin/gosu
gosu-0.10.2/bin/gosu: line 52: /Library/Internet: No such file or directory
gosu-0.10.2/bin/gosu: line 52: exec: /Library/Internet: cannot execute: No such file or directory
$ 

// expected result: Command prompt is displayed.
$ ./gosu-0.10.2/bin/gosu
Type "help" to see available commands
gs>