Cherry-pick phobos commit to make std.math unit test IEEE 754 compliant
Do not allow failures for LLVM 3.4 on Travis-CI.

LLVM 3.4rc2 has been released. As the last issue is fixed, we now start to
support LLVM 3.4.
Adds also a build for LLVM 3.5 (which is allowed to fail).
Merge branch 'master' into merge-2.064

Conflicts:
	runtime/phobos