"dmd -O" does not support Mir Algorithm

[Ilya Yaroshenko]

Hi,

I added unittest-release builds into Travis. LDC works well but 
DMD fails in few places.

One DMD bug was filled [1]. I will explore others when this is 
fixed. Please use only LDC for builds with O flag for now.

[1] https://issues.dlang.org/show_bug.cgi?id=17943

Best regards,
Ilya