mean and var test failing sometimes

Most of the time (although not always) the `mean` and `var` tests fail:

```
...v0.4/ArrayFire/test(master) >> julia
_
_ _ _(_)_ | A fresh approach to technical computing
(_) | (_) (_) | Documentation: http://docs.julialang.org
_ _ _| |_ __ _ | Type "?help" for help.
| | | | | | |/ _` | |
| | |_| | | | (_| | | Version 0.4.5 (2016-03-18 00:58 UTC)
_/ |\__'_|_|_|\__'_| |
|__/ | x86_64-unknown-linux-gnu

julia> include("runtests.jl")
Device[0] has no support for OpenGL Interoperation
ERROR: LoadError: test failed: 0.090069480240345 == 0.090069495f0
in expression: var(ad) == var(a)
in error at ./error.jl:21
in default_handler at test.jl:28
in do_test at test.jl:53
in include at ./boot.jl:261
in include_from_node1 at ./loading.jl:320
while loading /home/mauro/.julia/v0.4/ArrayFire/test/runtests.jl, in expression starting on line 47

julia> include("runtests.jl")
ERROR: LoadError: test failed: 0.5107571f0 == 0.51075715f0
in expression: mean(ad) == mean(a)
in error at ./error.jl:21
in default_handler at test.jl:28
in do_test at test.jl:53
in include at ./boot.jl:261
in include_from_node1 at ./loading.jl:320
while loading /home/mauro/.julia/v0.4/ArrayFire/test/runtests.jl, in expression starting on line 42
```

Also note that it is a bit strange (wrong?) that `var` returns a double.

I'm running on the built-in Intel HD graphics using [Beignet](http://arrayfire.com/opencl-on-intel-hd-iris-graphics-on-linux/) but it also happens using the CPU backend.