Resizing Macintosh HD partition to use free space
This Ask Different post has what you're looking for. Don't go looking for an "answer," the author for some reason decided to write his own answer in the "question" post. The method worked perfectly (and non-destructively, but you should always take a backup anyway, just in case) for me! 

Paraphrased:


  
  Reboot in Recovery Mode by holding down Command-R as your Mac starts up.
  Open Terminal (from the Utilities section of the Menu Bar).
  Run the command, diskutil cs list
  Take the Logical Volume ID (a long string of letters and numbers) 10 lines up from the bottom of that command's output, and use it in the command:
  
  diskutil cs revert [Logical Volume ID]
  This should convert the CoreStorage Logical Volume to a normal disk partition.
  Run diskutil cs list again, and you should receive a "No CoreStorage logical volume groups found" message.
  Now you should be able to resize your partition normally through Disk Utility.
  


Good luck!