Wifi Keeps Dropping Macbook Pro 13in Retina Early 2015
I was having a similiar issue with 10.10.3 which was not corrected when I updated to 10.10.4. Many, many things were tried before I got it to stay connected to WiFi reliably, however deleting the networking PLIST files seems to be what finally did it.

The Fix:


Disable WiFi
Open Finder
Press Cmd+Shft+G to go to the folder /Library/Preferences/SystemConfiguration/
Delete (or move) the following files:


com.apple.airport.preferences.plist
NetworkInterfaces.plist
preferences.plist

Restart your computer
Re-enable WiFi


The Other Things I Tried:


Reset PRAM
Reset SMC
Used Onyx to clear caches, run update scripts, etc.
Disabled AirDrop by using Terminal to run sudo ifconfig awdl0 down
Kext Utility to reset kext permissions, followed by...
Disk Utility to verify/correct permissions. 


In my case errors were found, and I had to...
Reboot in Recovery Mode (press Cmd+R as boot sound, hold until you see the Apple logo), use Disk Utility, however no errors were detected
Disabled FileVault, waited for decryption to complete
Reboot in Recovery Mode, use Disk Utility, some errors corrected then return value 8 and was told I had to backup, format, and restore to the drive.
Ran Disk Utility repair again and no errors were detected (then I ran it again for good measure)
Rebooted and Disk Utility found no errors.
Still had WiFi disconnects...

Deleted the AirPort plist files (fix above) and WiFi has been stable.