Fix Libdl moved to stdlib on v0.7

[blegat]

Closes #159

[chengg04]

Added those changes to my local branch, but got the following error while running Pkg.test("CPLEX"):

ERROR: LoadError: UndefVarError: Libdl not defined
Stacktrace:
 [1] top-level scope at /Users/chengguo/.julia/v0.6/CPLEX/src/CPLEX.jl:8
 [2] include at ./boot.jl:292 [inlined]
 [3] include_relative(::Module, ::String) at ./loading.jl:521
 [4] include(::Module, ::String) at ./sysimg.jl:26
 [5] top-level scope
 [6] eval at ./boot.jl:295 [inlined]
 [7] top-level scope at ./<missing>:2
in expression starting at /Users/chengguo/.julia/v0.6/CPLEX/src/CPLEX.jl:8

Then added using Libdl to CPLEX.jl, but the above problem persists.

The system is MacOS.

[blegat]

What does julia -e 'println("$VERSION")' print ?

[chengg04]

0.7.0-DEV.3444

[blegat]

The 3 lines added in this PR need to also be added to CPLEX.jl

[blegat]

@g001234 What about now ?