Remove remaining mod.

[helgee]

One last reference to mod_fast slipped through and broke precompilation, like this:

julia> using Unitful
INFO: Precompiling module Unitful.
ERROR: LoadError: LoadError: KeyError: key :mod not found
Stacktrace:
 [1] getindex at ./dict.jl:474 [inlined]
 [2] macro expansion at /Users/helge/.julia/v0.6/Unitful/src/fastmath.jl:155 [inlined]
 [3] anonymous at ./<missing>:?
 [4] include_from_node1(::String) at ./loading.jl:539
 [5] include(::String) at ./sysimg.jl:14
 [6] include_from_node1(::String) at ./loading.jl:539
 [7] include(::String) at ./sysimg.jl:14
 [8] anonymous at ./<missing>:2
while loading /Users/helge/.julia/v0.6/Unitful/src/fastmath.jl, in expression starting on line 154
while loading /Users/helge/.julia/v0.6/Unitful/src/Unitful.jl, in expression starting on line 1071
ERROR: Failed to precompile Unitful to /Users/helge/.julia/lib/v0.6/Unitful.ji.
Stacktrace:
 [1] compilecache(::String) at ./loading.jl:673
 [2] require(::Symbol) at ./loading.jl:460

[ajkeller34]

Ack, you're kidding! Thanks for the PR.

[coveralls]

Coverage remained the same at 67.88% when pulling 15ec882 on helgee:patch-1 into ead2d96 on ajkeller34:master.