app-misc/powerline: remove deprecated dependency on python 2.6 and add

Python 2.6 has been removed from the tree and therefore the ebuild should not depend on python_targets_python2_6

I've added python_targets_python3_4 instead (works for me).

The actual error I get is:

!!! All ebuilds that could satisfy "powerline" have been masked.
!!! One of the following masked packages is required to complete your request:
- app-misc/powerline-9999-r8::raiagent (masked by: invalid: DEPEND: USE flag 'python_targets_python2_6' referenced in conditional 'python_targets_python2_6?' is not in IUSE)

For more information, see the MASKED PACKAGES section in the emerge
man page or refer to the Gentoo Handbook.