projectile-kill-buffers: Ignore indirect buffers

When an indirect buffer is located after its base buffer in the buffers
list, `projectile-kill-buffers' gives an error ('No buffer named ...').
This commit filters out indirect buffers, which will be killed when the
base buffer is killed.
Merge pull request #360 from kyleam/fix-kill-buffers

projectile-kill-buffers: Ignore indirect buffers