Partition SSD in Yosemite for Dual-Booting Ubuntu
You have to boot from another device or to Internet Recovery Mode and then use the command line to modify the CoreStorage volume.

Preparation:  


Backup your data.
Detach any external drive (especially your external Time Machine backup drive).
Restart to Internet Recovery Mode by pressing alt cmd R at startup.
The prerequisites are the latest firmware update installed, either ethernet or WLAN (WPA/WPA2) and a router with DHCP activated.
On a 50 Mbps-line it takes about 4 min (presenting a small animated globe) to boot to a recovery netboot image which usually is loaded from an apple/akamai server.

I recommend ethernet because it's more reliable. If you are restricted to WIFI and the boot process fails, just restart your Mac until you succeed booting.

Alternatively you may start from a bootable installer thumb drive (Mavericks or Yosemite) or a thumb drive containing a full system (Mavericks or Yosemite).


Modify CoreStorage:


Open in the menubar Utilities/Terminal
Enter diskutil cs list
Enter diskutil cs resizeStack LVUUID size with LVUUID: UUID of the logical volume and size: the final size of your Macintosh HD
Example: diskutil cs resizeStack DC2473AF-0833-4DBD-BF8F-D5D60063554A 200g  
Enter exit and quit Terminal.
Reboot to your main volume.
Start installing Ubuntu to the now unallocated disk space.