[OSX] fix a wrong default deployment_target on OS X 10.8
[OSX] fix a wrong default deployment_target on OS X 10.8

http://hipbyte.myjetbrains.com/youtrack/issue/RM-524

'10.8' should be used as default deployment target on OS X 10.8.
However, '10.9' was used.

katsuyoshi/RubyMotion,jjaffeux/RubyMotion,beni55/RubyMotion,HipByte/RubyMotion,hboon/RubyMotion,beni55/RubyMotion,jjaffeux/RubyMotion,incbee/RubyMotion,katsuyoshi/RubyMotion,jjaffeux/RubyMotion,katsuyoshi/RubyMotion,incbee/RubyMotion,HipByte/RubyMotion,incbee/RubyMotion,jjaffeux/RubyMotion,HipByte/RubyMotion,hboon/RubyMotion,beni55/RubyMotion,beni55/RubyMotion,incbee/RubyMotion,katsuyoshi/RubyMotion,hboon/RubyMotion